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Go for the HD 4850. But if your going to be gaming your making a huge mistake getting a mac. They don't support many games even when they are running windows. You should build yourself a gaming pc. I can tell you from experience for the money you spend on the mac you can get a pc that has alot higher specs and is much more capable of doing what you want

Anytime the phrases "Mac" and "heavy gaming" appear in the same sentence, the word "not" or similar must also be used. I do not see this. Please correct.

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.

4850. And why are you buying an iMac??? Have you never heard of a Hackintosh??? 10 times cheaper and WAY better.