Bought a higher-end 2010 iMac off a friend who was moving and didn't want to take it with him. My 2011 MacBook Pro wasn't keeping up with the few games I wanted to play via BootCamp (Skyrim, Cities: Skylines, Diablo, etc.), so I figured the better specs on the iMac would help keep me from needing/wanting to buy a new computer in the next twoish years. I knew that it was starting to experience some artifacting and kernel panicking due to the video card starting to go out, but for $250, I wasn't going to say "no"! I figured I'd be able to repair it myself when the time came it was unusable. So, when it did finally go, I found a video card on eBay for $200 and sat down to replace it.
The repair went really well, overall! I used iFixIt's guide to remove the front glass panel and then the display panel so I could get at the video card (found here: iMac Intel 27" EMC 2390 Display Replacement). I then used this guide from teut986 (found here: iMac Intel 27" (Late 2009 or Mid 2010) Graphics Card Replacement) to walk through the work of taking out the other innards needed to access and replace the video card. I was a little intimidated by something like the iMac. I'd only taken Apple's laptops apart before and I had never had to go as deep into them as I did the iMac. That said, though, overall the process was really, really easy. Overall, I'd say it took 60-90 minutes, but half of that was getting REALLY frustrated trying to put the GPU heat sensor cable plugged back into the man logic board. It's really, really small, has to be aligned a specific way, and goes in on the back side of the board, in the tightest spot possible with very little room for maneuvering. But, that's the only difficult part. Everything else was really straightforward and easy-to-do!
1) Maybe get some tweezers to help with the GPU sensor cable. The pair I had didn't really help too much, but you might have some worth a %#*@ to try out. I finally ended up getting it by sheer blind luck and a weird "from behind" angle where I had to do it by-feel as I couldn't see anything.
2) Set up a space with something LINT-FREE to set the panel glass and display panel on and then cover them both immediately with more LINT-FREE material. I used newspapers and it worked pretty well.
3) Do not let dust get between the glass and the display panel. You will think "I can ignore that. It's not a big deal" for about 10 minutes until it starts to drive you crazy and you're opening it back up to get the canned air and such in there to fetch it out.
4) Consider wearing some gloves while handling the glass and display. You don't want fingerprints on the inside (see 3)
5) An empty, dry, ice tray works REALLY well for holding and keeping all those screws organized
Heavy-Duty Suction Cups (Pair)
TR10 Torx Security Screwdriver
TR9 Torx Security Screwdriver