Don't Do It!
Just buy a Mac and if you want to play games, get a Playstation, Wii, xBox or whatever.
Modern Macs don't do terrible at playing most "reasonable" games. They do as well or better than most "normal" un-modified store bought PCs.
The big issue is finding high-end video games written for Mac OS!
Building a "Custom high-end Gaming PC" isn't going to be a lot less than any Mac I can think of....
Don't those usually have Quad-Core i7 CPUs, OC-inspired motherboards, Kilowatt Power Supplies, multiple real expensive video cards, 10,000rpm or solid-state Hard Drives, gobs of memory and of course a case shaped like a Transformer's head? Oh and blue lights.
Lots of blue lights.
Seems to me I could almost buy a nice Octo Mac Pro for the cost of all that junk. A new Mac, even a Mini will probably hold its value better than a PC too.
Have Fun!
Keri
PS. Try Machinarium!
C&C Red Alert 3 works pretty good too. (on a Mac Pro, that is) Well, I think it does. It seems to beat me very smoothly.
(added) PPS. I don't intend to imply that a dedicated Gaming PC is a bad thing. That's the way to go for serious Gamers.... for now.