My 2008 iMac was lagging and I mean lagging but after 6years I figured I'd have to bite the bullet and buy a new one. I was at an Adobe software meet up and the Adobe rep described how he put an SSD in his 2008 iMac and it was like having a new machine. As usual I turned to iFixit before ordering any parts and sure enough I easily found your guide to replacing the HD in a 2008 iMac.
I followed the iFixit guide step by step and had no problems. You, must buy the suction cups though as there is no other way to get the screen off. I have heard that a toilet plunger will work but i just couldn't do that to a Mac, so for $15 i have a nice set of suction cups that I can help my friends to upgrade too and keep their iMacs running longer. Thanks as alway for great documentation.
This is not for anyone who has never opened up a PC. Usually i just upgrade the HDs in our laptops but this is CONSIDERABLY more complex. Enlist a friend or take it to a local computer repair who is comfortable doing this and maybe you could watch and learn.
If you have a 2009 or newer iMac you might want to get a kit from OWC and turn your DVD slot into another HD bay. They also sell a enclosure for your DVD to use as an external device but does anyone use DVD anymore.