Most ATA hard drives will fit in a 3.5" opening. They usually come with an adapter kit so they can fit in the 5.25" openings as well.
Personally, I like Seagate the best. Western Digital and Maxtor aren't too bad either. I would however stay away from most off brand name ones. I have seen a lot of them go bad before their time.
What do you do with your pc? If you do any audio or video I would try to get a hd with the folowing specs:
and check out seek times as well
I haven't bought a new HD in a bit so there may be better than this out there now but that'* my 2 cents.