I would suggest something like this...
Parts
CPU: Intel Core i7-930 Processor Boxed
http://www.newegg.com/Product/Produc...-225-_-Product
Mobo: GIGABYTE GA-X58A-UD3R LGA 1366 Intel X58 SATA 6Gb/s USB 3.0 ATX Intel Motherboard
http://www.newegg.com/Product/Produc...D=3463938&SID=
Graphics: Sapphire Radeon Toxic HD 5850
http://www.amazon.com/Sapphire-Rad [...] B003D0IS3K
PSU: Thermaltake TR2 TRX-750M 750W ATX 12V v2.3 / EPS 12V v2.91 80 PLUS Certified Modular Active PFC Power Supply
http://www.newegg.com/Product/Produc...D=3463938&SID=
HD: Samsung Spintpoint F3
http://www.newegg.com/Product/Produc...-185-_-Product
RAM: G.SKILL Ripjaws Series 6GB (3 x 2GB) 240-Pin DDR3 SDRAM DDR3 1600 (PC3 12800)
http://www.newegg.com/Product/Produc...D=3463938&SID=
Case: COOLER MASTER RC-692-KKN2 CM690 II Advanced Black Steel ATX Mid Tower Computer Case
http://www.newegg.com/Product/Prod [...] 6811119216
I took this from elsewhere, but I liked the build for the cost, so I just altered it a bit.
This build would pretty much blow 99.9% of games out of the water.
I am a big fan on intel which is why I picked the i7 as opposed to something more cash efficient.