iMac 6.1 Hard drive on its way out? What to replace it with?

Recently updated from tiger to snow leopard, upgraded the ram (2 x 2gig Kensington), and performance was still painstakingly slow. Then I started getting the blue screens and after struggling with that for weeks I finally gave in and did a complete erase of my harddrive, and installed snow leopard again. It worked fine for about a week, although it was still very slow. It froze in the middle of some software updates last night, so I did a hard reboot. It didnt recognize its own hard drive and would only boot from the osx install disk. Its only been a week since I erased the harddrive, so I dont mind doing that again. But if this is going to become a regular thing, i just might lose my mind. Any suggestions regarding what to do? What to replace it with? And is it fairly easy to replace the hard drive?

Appreciate any and all help or suggestions from u guys. This is so frustrating!

P.S. Oh, and by the way, whenever I install the OS, it takes like all night. And I forgot to mention, its a 24" iMac, core 2 duo ver. 6.1 from 2007

Message was edited by: prodigious1