I am sure that Apple will update the graphics side of the iMac's which would make things a little more even....
My Mac Pro (8800GT) runs Crysis Warhead on 3/4 of the highest quality without AA. Half-Life 2, Test Drive Unlimited, Devil May Cry 4 and GTA: SA all on the highest with AA. Gaming on the Mac Pro works fine, especially for CPU intensive gamesI was in exactly the same boat as you before i got my Mac. I fully expected it not to run them as well as my PC etc. That was until i actually got my Mac and found that it ran every game just as well if not better than my PC used to.
Obviously it isnt built for gaming, but it is more than adequate for the vast majority of games and gamers.
I play crap loads of BF2, UT3, CS:S, EVE, Lineage 2, TF2 and have had no bother what so ever with running them on mainly high settings with my 8800GT Mac Pro