A typical desktop hard drive is 3.5" in form factor, so just look for that and it should fit just fine (a lot of the time HDDs will ship in the bare drive, so you may need some screws that normally come when you buy a case). You should have multiple 3.5" bays to add another HDD if you just want to add a new one in there instead of replace as well. It's really a simple plug in power, connect another SATA cable, and done. Sometimes having to format it later on to show up in your computer (easy).
As for the drives themselves, Western Digitals lineup consists of Caviar Black/Blue/Green/Red. Black is 'performance' drive, Blue is the typical consumer grade drive, a mix of performance and value, Green is quiet and efficient, but also slower and usually meant for storage. Red is more meant towards servers/NAS and is "meant" to run 24/7.
There are also other brands that I know less of, like Seagate, Toshiba, etc. All of them should work fine as long as they are 7200RPM, etc.
Just make sure for a "regular" HDD:
3.5" form factor
Get necessary cables (SATA)