What type of RAM Should I Choose?
You must decide what type of RAM your homebuilt computer will use before you buy a motherboard. This is because most motherboards are able to use only one type of RAM.
Ordinary SDRAM is rapidly becoming obsolete. So if you are building a new computer, chances are you will be deciding between DDR-SDRAM or RAMBUS. But because RAMBUS is way expensive, you'll probably be using DDR. I suggest that you use DDR2 or DDR3 in dual-channel. It's fast, stable, and relatively cheap.
When using dual-channel DDR, pairs of identical RAM modules work in tandem to greatly improve performance. Both the size and the speed (and preferably, the manufacturer, in my experience) of each RAM module in a dual-channel pair must be identical.
If you're building a machine around the Intel Core i7 processor, then you'll certainly want to use DDR3 RAM, which is capable of data transfer rates twice those of DDR2.
How Much RAM do I Need?
Dollar for dollar, nothing will liven up your homebuilt computer more than endowing it with a respectable amount of RAM. In addition, because RAM is so central to your computer's functioning, always choose RAM made by a reputable manufacturer like Crucial.
Every operating system has "minimum system requirements" for processor speed and RAM; but in our experience, the minimum requirements tend to be grossly inadequate if you actually want the computer to do anything useful. Instead, we suggest the following RAM amounts for optimal performance of desktop PC's running the following popular operating systems:
Windows 98/Me: 256 to 512 MB
Windows 2000/XP: 1 GB to 4 GB
Windows Vista / Windows 7 32-bit: 2GB to 4 GB
Windows Vista / Windows 7 64-bit: 6 GB to 16 GB
Ubuntu (or other Linux with X-Server) 32-bit: 1 GB to 4 GB
Ubuntu (or other Linux with X-Server) 64-bit: 4 GB to 16 GB
If you use your computer for graphics manipulation, video editing, TV or DVD viewing, gaming, CAD/CAM applications, or complex mathematics, then you should lean toward the higher numbers. If you mainly use it for email, web surfing, and office apps, than the lower numbers should be sufficient.
There does come a point when you are not likely to see any noticeable improvement by adding more RAM. Going very much above the higher figures in the table above is not likely to improve your computer's performance in any noticeable way. And of course, you cannot install more RAM than your motherboard can support; so make sure you know this figure before buying up every last stick of RAM in the store!
Finally, if you're using a 32-bit operating system, then don't bother installing any more than 4 GB of RAM. A 32-bit system can only address about 3.5 GB, so buying anything above that is just wasted money.
The speed of RAM you will need depends mainly on the motherboard. You generally should choose the highest speed of RAM that the motherboard supports. Make sure that you check the motherboard documentation to find this out. Just because a stick of RAM fits in a motherboard doesn't mean it will work. Sometimes, inserting the wrong RAM and powering up the mobo can damage the board, the RAM, or both.
Even if your motherboard can support different RAM speeds, using slower RAM than the fastest that the mobo supports means you won't get the maximum performance that the board is capable of delivering.
As the speed capabilities of RAM of a given type improve, newer, faster RAM is usually backward-compatible to slower speeds. This is accomplished through the use of a serial presence detection circuit built into the RAM module.
So for example, most PC-2700 (333 MHz) DDR-SDRAM sticks are backward-compatible to PC-2100 (266 MHz). So if your board is built for PC-2100, but you can get PC-2700 for the same price, you may as well go for the faster RAM. You never know: You may want to use it in your next project. (Just make sure that the RAM you buy is backward-compatible to the speed you need now!)
The one thing you generally should avoid, however, is mixing RAM speeds in the same computer. In theory, if you mix RAM speeds, all of the modules should clock to the lowest speed; but in practice, mixing speeds (and sometimes even brands) in the same PC can cause system instability. (And of course, RAMBUS and dual-channel DDR must use matched pairs of identical modules.)
So in summary, even though different brands of RAM (and to an extent, different sizes and speeds, except for RAMBUS) can theoretically be mixed, I continue to recommend using identical RAM sticks in any given computer to maximize stability and minimize the possibility of conflicts.