Not a problem actually. Just wanted to upgrade my iMac 27 late 2009 ATI HD 4670 256Mb (low end) with a much better 4850 512Mb card.
All the process went straight forward, the only problem was the sync ribbon LCD cable which is anything but tough. I had to straighten it a bit with a pair of tweezers before I put it back.
Watch out with the sync LCD cable, it is very fragile!