Absolutly. You can buy a official one on the Apple Store. I reccommend the dual-drive (HDD & SDD) tho.
FAN SPEED PROBLEM -> FIXED FOR FREE
When replacing HD with same (newer model) type and brand the fan-full-speed problem still occurred. Reason: All HD's not sold by Aplle Store don't have required special apple firmware to handle the Apple built-in fan controls. Fix: FREE program: http://www.crystalidea.com/products?ref=... Don't spend money.
OVERHEAT PROBLEM / CLICKING HD
My medio 2001 iMac heats more then box windows pc: large screen that generates heat very close to components. When you hear the HD "click click", it's safe to say your HD is running too hot. Play with settings in Mac Fan Control prog to find the "no click"-setting. Check manual HD.
SLOW MAC = HOT MAC = BURNING HARD DRIVE TOO MUCH
Macrunning slow? check available RAM in Activity Monitor. If it's less then 1 gig, by some more. Dead cheap at +- 50$ / 4 gig.
EU USERS OF IFIXIT
1) Google for iFixIt Europe to find a German based webstore.
2) Buy kit + the screwset, the screws are USA type.
It's dead easy, took me 30 min.