DO NOT BUY THE MAC!!! I am actually Apple Certified and have seen a lot of the 20 inch versions of the new iMac. We've got 150 or so of these, and so far have had a decent amount of problems. I work for a school district, and my boss in fact said he is not buying another 20 inch iMac. Heating issues, computer sometimes shut off at random, Apple claims replacing a cable may resolve some of this, but a lot of the time, if you need say a logic board replacement, you are looking at almost the price of the machine for repair.
So in short, do not get that iMac. If you want to game, get a pc. Again, this is from someone who actually has their Apple Certification.