$169 Intel Core 2 Duo E6550
$142 Patriot Viper 4GB(2 x 2GB) 240-Pin DDR2 SDRAM DDR2 1066 (PC2 8500)
$219 XFX GeForce 8800GT PVT88PYDF4
$109 Antec NeoPower 650 650W Power Supply
$119 ASUS P5N-E SLI LGA 775 NVIDIA nForce 650i SLI ATX Intel Motherboard
$69 Western Digital Caviar SE16 250GB 3.5" SATA 3.0Gb/s Hard Drive - OEM
$827

buy a case if you can fit it in the budget.