Network Cards and Modems
A network card (also known as a Network Interface Card, NIC, Network Adapter, or Ethernet card) is used to connect a computer to a high-speed network. If your homebuilt computer will be on a local network (or if you ever plan to use DSL, cable, FIOS, or satellite Internet service) then you should install a NIC if one isn't built into the mobo.
High-quality networks cards are inexpensive enough that it really doesn't pay to skimp. The five bucks or so that you may save by using an el-cheapo NIC isn't worth having to drive (or ride your bike) to the computer store, buy a new one, re-open your computer case, and install new drivers when the cheap NIC fails.
Most NIC's today still use the 10/100 Mbps Fast Ethernet protocol, but the 1000 Mbps Gigabit Ethernet standard is catching on. Right now, Gigabit Ethernet cards are still a bit more expensive (no pun intended) than 10/100 Mbps cards; but as their prices continue to fall, Gigabit Ethernet will become the new standard. Most new mobos have them built in.
Almost any NIC will come with drivers for all recent Windows versions. If you are planning to install a different operating system like Linux, BSD, etc., then make sure your NIC will work with the OS you plan to use.
The word "modem" is short for "modulator/demodulator." A modem converts your computer's digital data to analog data that can be sent over a POTS (plain-old telephone service) line, and converts incoming analog data to digital data that the computer can work with.
Not many people use dial-up modems anymore. They're used mainly by people who need them because they have no alternative other than dial-up (usually because they live in the boondocks where broadband isn't available), or because they have to connect to networks that require dial-up connections for security reasons. So if you don't have any of these needs, then don't waste your money on a modem.
Internal PCI modems can be had for as little as five bucks. But just as with network cards, you get what you pay for. Most cheap modems are not really modems at all. Rather, they use software and your computer's system resources to modulate and demodulate data.
Better-quality modems have built in controllers that do the work right on the card. They produce a faster, more stable, and more reliable connection than do cheap software modems.
External modems that connect to a computer's serial or USB port are also available.
As with NIC's, if you are planning to install an operating system other than Windows, then make sure your modem will work with the OS you plan to use before you plunk down any money for the modem.
Integrated NIC's and Modems
Many motherboards have integrated (built-in) network cards and/or modems (although modems are becoming less common as fewer people are using dialup Internet service). The quality of these built-in adaptors range from truly horrible to quite good, depending on the quality and price range of the board.
If you select a motherboard with a built-in modem or NIC, make sure that there are expansion slots available for an aftermarket card, just in case the on-board device ever fails or doesn't work to your satisfaction. And yes, once again, remember that on-board devices are designed with Windows in mind, and they may not work with other operating systems.