Choosing a CPU
Choosing a CPU for your homebuilt computer is one of the very first things you will want to do during the planning stage. The CPU you choose will affect almost every subsequent decision you make about the components you choose, and will be the most crucial factor in determining your computer's abilities and limitations. So the first step in choosing your processor is knowing what you want the computer to do. If the tasks you need it to do are simple such as word processing, reading email, and surfing the Web, then you don't need a bleeding-edge processor. But if you're a gamer or do video editing, use CAD/CAM software, or do complex mathematics, you'll need to aim higher.
Things to Think About when Choosing a CPU
Number of Cores
Of particular interest to most computer builders is the number of cores, so we may as well start here.
The "core" is the actual CPU that is mounted on a processor chip. It's the part that actually crunches numbers, which is all any CPU actually does when you get down to it. Originally, processors had one core, and high-end servers and other computers that needed more processing power would employ two or more processors.
Then one day processor manufacturers got the bright idea of mounting two cores on one chip, providing more processing power at lower cost and in a smaller package. And so we had the Athlon 64 X2 series on the AMD side, and the Core 2 Duo series on the Intel side. Each core on a multi-core chip is recognized as a CPU by the computer, so dual-core chips represented a major leap ahead for ordinary users: They could now enjoy the advantages of simultaneous multi-processing at a fraction of the cost of dual-processor machines.
Multiple-core CPU's are now the norm rather than the exception. You can't swing a cat without hitting a quad-core or hex-core processor, some at very reasonable prices. Even octa-core processors have become commonplace and are very popular among gamers, scientists, architects, and others with extreme processing needs. You can even combine multiple, multiple-core processors on the same motherboard. For example, two octa-core processors on a dual-processor motherboard will result in a 16-processor machine.
But will it be faster?
The answer is that it depends. Having more cores is like having more workers on a job site. If there are more workers than there is work to do, then adding even more workers won't speed things up. But if there is more work to do than there are workers to do it, then hiring more workers will get the job done faster.
Let me use myself as an example. The machine I'm using right now has an Intel i7 6700 processor, a quad-core processor with hyper threading (so it appears as eight processors to the machine). The motherboard is capable of supporting a processor with more cores, but if I upgraded, I'd never notice the difference. Most of what I do on this machine is banging out code, which is just text, and which puts little strain on a processor. So I could go whole-hog and install four eight-core processors for a 32-core system, and still not notice any difference in performance.
For a while, however, one of my clients was sending me a huge number of videos to edit. Now that was the kind of work where more cores could have sped things up. I thought about upgrading my processor, or maybe even building a new computer around one of the exciting new AMD Ryzen Threadripper processors. But then the videos dried up; and as much as I love building computers, I'm not going to spend all that money building a machine that won't make any difference given the way I use it.
At the time of this revision (in February of 2019), my feeling about how many cores a user needs is the same as it was the last time I revised the site, and that is that for average users, I think a good quad-core processor is still the sweet spot as far as performance per dollar is concerned. For gamers or others who use resource-intensive software, I'd recommend looking at some of the octa-core processors like the Intel Core i7-9700K if you prefer Intel, or the AMD Ryzen 7 2700X if you prefer AMD.
The socket architecture of a CPU refers to several things, the most important of which for the do-it-yourself computer builder is whether the CPU will fit in the socket and be supported by the accompanying architecture. You must purchase a motherboard that supports your processor's socket requirement, otherwise it simply won't fit. (Conversely, if you're upgrading an existing computer or using a mobo you already have, you must purchase a processor that your mobo supports.) The socket also determines the type of CPU cooler you'll need, although there's a great deal more standardization in this area.
The socket architecture means a lot more than whether the processor will fit in the socket, however. The socket architecture is part of an overall set of hardware specifications that affect performance and are bound to change with progress. Older form factors are therefore retired at some point when they can no longer support the most bleeding-edge processor technology.
What this means for the computer designer (who would be you) is that you have some choices. If your computing needs are pretty basic, you probably can save some money by using a processor based on a "mature" socket architecture like the Intel LGA 1151. The i5-7640K and i7-7740K processors use the newer LGA 2066 socket, which probably means the end of development of 1151-compatible processors. If your computing needs are basic, you may be able to save some money on an LGA 1151 motherboard precisely because those who want the latest and greatest won't be interested in a mobo based on a socket that's nearing its sunset.
But if you're a gamer or other power user who will always want to be on the bleeding edge hardware-wise, then you're probably better off leaning toward a "younger" socket architecture like AMD's AM4 or Intel's LGA 2066. They're more future proof. An AM4 motherboard on the AMD side, or an LGA 2066 motherboard on the Intel side, are muchmore likely to support future bleeding-edge processors than an LGA 1151 motherboard would be.
The speed of a CPU determines how many computations it can perform per second. (In the case of a multi-core CPU, it determines how many computations each core can perform in a second.)
Obviously, a faster chip can perform more computations. If you plan to use your computer only for pretty routine, low-resource applications like surfing the web, word processing, and checking email, you can save yourself some money by buying a somewhat slower chip (for example, one or two notches below the top of that line of processors, or maybe last-year's model of the processor manufacturer's flagship line). If you're a gamer, do video or music editing, or use your computer for high-end graphics or CAD/CAM apps, on the other hand, then set your sights higher.
Onboard cache bridges the gap between the very fast CPU and the much slower system RAM bus by anticipating the processor's data requests and storing that data right on the CPU itself. This dramatically increases performance at a given clock speed. All else being equal, more cache is always better.
L1 and L2 cache are dedicated to individual cores and store the data for the cores' current and next tasks. L3 cache is shared between the cores and stores the data that is likely be required by any of the cores or by more than one core. That's why sharing L3 cache can improve performance, whereas sharing L2 cache between cores would degrade performances.
Many processors in both Intel's and AMD's lines have graphics processors built right into the CPU. As with so many other decisions to make when designing a computer, whether this is a good thing or not depends on how you plan to use the computer.
Either company's integrated graphics will do a good enough job for things like watching movies on your computer, photo editing, and possibly even light video editing if the processor is beefy enough to do the heavy lifting and if you have enough system RAM. But if you're a gamer or other user of graphics-intensive software, my guess is that you will not be happy with integrated graphics as they exist today.
If you're building a computer that will be used for graphics-intensive, work-related applications that will require a high-end video card, and reliability is paramount for you, then you may want to consider going with a server CPU like Intel's Xeon series or AMD's EPYC series. Server processors and motherboards designed for them are stable, powerful, reliable workhorses that thrive on prolonged, intensive use. If I were designing a computer for an architect, draftsman, professional video editor, or someone else who needed high-end graphics and rock-solid reliability, that's how I'd be leaning.
As with any major purchase, do your homework before buying. Check Internet message boards and forums to see how satisfied other users have been with the chips you are considering, and read the reviews often posted on retailers' sites. If you're a gamer or use other resource-intensive software, check with forums dedicated to the games or software that you use, or check with the software publisher for their recommendations. Some software works better with some chips than it does with others.
Intel vs. AMD
This is one of the first questions a computer builder usually ponders, unless they've already developed a loyalty for one company over the other. It's not a question with a simple answer. Some people swear by one company's products or the other, while others look only at the numbers. Skinny armed, pimple-faced geeks have gotten into fist fights over which company's processors are better.
My take on the question as someone who has used both companies' chips is that both AMD and Intel make excellent chips toward the top of their respective lines. AMD, being the underdog, tends to be more on the bleeding edge than Intel, which has resulted in their chips sometimes having the early edge over Intel's. They also tend to be less expensive at most ranges of performance.
On the other hand, my experience has been that Intel's somewhat more conservative approach to processor development gives them a bit of a stability advantage. Their rather stodgy nature may result in chips that are less exciting, but few people would argue that they're not stable (especially when using a motherboard with a genuine Intel chipset).
Consequently, when building machines for my own use, I've often gravitated toward AMD because of their lower cost at a given performance level and their tendency to be a little more aggressive in seeking the bleeding edge. But when building machines for customers (especially businesses), or machines for which reliability is my number one concern, I've always leaned toward Intel because I've found the Intel architecture to be rock-solid stable.