I've installed ATI cards of various designs into cases of various sizes, (all PC), since '96 without ever having any problem. However, I would not be surprised if there is a problem fitting them into Macs.

Had four capacitors blow up on my 256mb card. Friend brought over a 256 card from his machine (he had upped to a 512). Card would not fit; had to cut the case. Installed card - not working. Finally got a replacement card; different brand but fits perfectly. Everything's fine now, except for the gaping hole that I cut in the back of the case.

Seems there are two sizes of cards, with the difference being between the edge of the card and it's attachment to the back of the case. Best way to test for fit might be to simply compare the cards to see if they have the same footprint.