Casing
Usually, casings come in either desktop or tower types. The desktop type has the side with the largest area as the base and the motherboard is installed at the bottom.

There are basically three tower sizes, the mini tower, medium tower and full tower. Most PCs come in mini tower to save space, but if you have a lot of devices to fit on it, it's better you get a bigger casing. Oh yeah, you have to make sure you get a bigger power supply as well to accommodate your extra devices.

There is also an ATX form factor casing for ATX motherboards. ATX is just a standard that crams a lot of stuff in one small place and supposedly improves functionality or the sort.

Whatever you choose, make sure your components can fit in and cables are usually supplied with it.

Port
Most motherboards support two serial and one parallel ports. So make sure you get the correct one for your printer, modem and mouse. There are several kinds so if you're not sure, simply compare the various cables of the devices with the ports and see if they fit. Easy?

If your sound card doesn't have a game port for that joystick you wanna use, get a game port (or change your sound card). Do I have to teach you everything?
 

Modem
Modems are really hot these days and it's all because of the internet craze. If you can't afford ISDN or any of those costly methods of surfin' the web, you gotta get a modem (else how're you gonna read this fabulous site again?). Before you get a modem, look for the V.34 and V.42 bis protocols. These are widely accepted standards that will ensure you don't tear the hair out of your scalp when your modem is not supported.

Some modems include voice features and you can use it like an answering machine. Some include fax capbilities. Some have both! Whatever the case, you have to know that the current situation is divided between support for the technologies from two companies. The Two technologies are X2 and Flex. They both gives the same speed, but are two different types.