Just for more variety I post an AMD build, just for the record I know little of AMD's besides looking at them for the past day or 2 so if you do go with what I post get it double checked by someone else. And I just slaped this build together in a few minutes last night just to get a rough idea of what it would cost me to do an upgrade. Oh and if you look around you can probably find better deals + bundles.
AMD Phenom II X4 810 2.6GHz 4 x 512KB L2 Cache 4MB L3 Cache Socket AM3 95W Quad-Core Processor - Retail $169.00
http://www.newegg.com/Product/Product.aspx?Item=N82E16819103650
ASUS M4A78T-E AM3 DDR3 AMD 790GX ATX AMD Motherboard - Retail $139.99
http://www.newegg.com/Product/Product.aspx?Item=N82E16813131366
Western Digital Caviar Black WD6401AALS 640GB 7200 RPM 32MB Cache SATA 3.0Gb/s Hard Drive - OEM $79.99
http://www.newegg.com/Product/Product.aspx?Item=N82E16822136319
SAPPHIRE 100258-1GL Radeon HD 4850 1GB 256-bit GDDR3 PCI Express 2.0 x16 HDCP Ready CrossFire Supported Video Card - Retail $161.99 (2 of these for crossfire)
http://www.newegg.com/Product/Product.aspx?Item=N82E16814102802
PC Power & Cooling S75QB 750W ATX12V / EPS12V SLI NVIDIA SLI Certified (Dual 8800 GTX and below) CrossFire Ready 80 PLUS Certified Active PFC Power Supply - Retail $99.99 after sale and rebates
http://www.newegg.com/Product/Product.aspx?Item=N82E16817703009
G.SKILL 4GB (2 x 2GB) 240-Pin DDR3 SDRAM DDR3 1333 (PC3 10666) Dual Channel Kit Desktop Memory - Retail $69.99
http://www.newegg.com/Product/Product.aspx?Item=N82E16820231190
Subtotal: $898.94
Tax: $65.17
Total: $964.11
But you still need a case, and accessories, but thats more personal preference so I leave that up to you. And if you plan to OC pick up a heatsink for the CPU I recomend one but I know nothing really AMD chips besides what I learned in the past few days which so far in my opinion they look pretty nice for price/performance wise.