IMO, the days of building PC's are long gone in terms of being cost effective. Those were the good ole days. It was a great passive income. If buying a gaming machine instead of building one is an option I would check out a Dell XPS 720 H2C. Alienwares are also nice but they have a nasty price tag.
I personally havent't gamed in quite a while but the lastest generation of games (UT3 for example) are insane when it comes to resource utilization. Be sure to get a beast and , in my opinion SLI is a must!!!
Good Luck!!!