AT has some good guides for building a gaming PC and I mostly agree with every part they pick with the exception of their cases (I go cheap, it's just a case, not a show piece). I always advise my customers that it's generally best to go middle-of-the-road when purchasing/building a new computer. High-end is way over-priced for little performance gains (looking at you Intel Extreme CPUs) while low-end will last you less than a year, if that.
The popular gaming build right now is Intel i5 2500k/Z68 motherboard with either a Radeon 6870/6950 or 560ti 448 core. It really depends on your monitor resolution and your preferred eye-candy.