Hey..
You just gotta pick the size you want..250,320, 500...etc..
Then if you have Sata--get a sata drive...if not not get an ide...
or if u have an adapter card for sata..you can still get a sata drive.
Sata is preferrable as it is faster and plug/play..
You also want one that spins at 7200rpm..
As for brands..big guys are Western Digital and Seagate but there are many offerings from a wide variety of players.
hope that helps a bit.
If you have a price range and know the size you like..we can suggest drives for u.