Would you consider a PC? I just got my MBP and I really like it, but I have to say, it is not economical for playing games. Though it is possible to configure a very powerful gaming Mac, you will be paying 3x-5x as much, for equal performance, as you would if you built a desktop PC.
Gaming is all about the graphics card, but Apple tends to neglect the graphics card in favor of a more powerful processor or better RAM.
I looked into building a PC, but I want a dual CPU system, I found it was cheaper to go with Apple. I'll be getting the GTX 285.