Ok here we go. You can get the cpu for $10 less at Microcenter and the motherboard for free (maybe not the same one, just pick a free or cheap one).
An intel i3 build would cost about the same so your choice.
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LOL didnt see that you didnt need a HD , optical drive, case, or psu. Next time you should say upgrade instead of build.
Anyways get a 7870 LE as django suggested instead.
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PCPartPicker part list:
http://pcpartpicker.com/p/FkvC
Price breakdown by merchant: http://pcpartpicker.com/p/FkvC/by_merchant/
Benchmarks: http://pcpartpicker.com/p/FkvC/benchmarks/
CPU: AMD FX-6300 3.5GHz 6-Core Processor ($119.99 @ Microcenter)
Motherboard: Gigabyte GA-78LMT-S2P Micro ATX AM3+ Motherboard ($49.98 @ SuperBiiz)
Memory: G.Skill Value 8GB (1 x 8GB) DDR3-1600 Memory ($29.98 @ Outlet PC)
Storage: Western Digital Caviar Blue 500GB 3.5" 7200RPM Internal Hard Drive ($39.99 @ Microcenter)
Video Card: Sapphire Radeon HD 7850 1GB Video Card ($159.99 @ Newegg)
Case: Zalman Z5 ATX Mid Tower Case ($25.98 @ Newegg)
Power Supply: Corsair Builder 500W 80 PLUS Bronze Certified ATX12V Power Supply ($39.99 @ Newegg)
Optical Drive: Samsung SH-224BB DVD/CD Writer ($17.98 @ Outlet PC)
Total: $483.88
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2013-02-22 22:38 EST-0500)