If you're willing to buy a new system, and build it yourself, you can get WAY WAY more performance for your cash than if you'd bought any mac system. It's a must if you want to seriously get into gaming, or even if you want a little more freedom from the cookie-cutter world of Apple.
I'm not saying Apple makes bad computers, but in my opinion, if you want to get the most bang for your buck, hand-built PC all the way. I'm sure I can come up with some examples if you'd like, as I'm currently in the process of planning a budget-gaming build for a friend.