Case:
Antec Sonata III w/ 500W PSU: $129.95 - $30 rebate + free shipping
CPU:
Intel Dual Core E2160 $67.99 + free shipping (and free Lost Planet game with code on webpage)
MB:
Gigabyte GA-P35-DS3L $87.75 + $1.99 shipping
GPU:
eVGA 8800GS 384MB: $139.99 - $30 rebate + free shipping
Memory:
A-Data 2GB (2 x 1GB) DDR2 800: $42.99 - $10 rebate + free shipping
HDD:
Seagate Barracuda 7200.10 320GB 7200RPM 16MB Cache OEM: $79.99 + free shipping
Optical drive:
Samsung Black DVD±RW Burner with LightScribe - OEM $26.99 + free shipping
OS:
Windows XP Home SP2B OEM $85.99 + free shipping
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Total = $593.63 -$70 rebates + $1.99 shipping = $525.62 + tax
Throw in another $20 for a basic keyboard/mouse. Note that with this system, you would really need to overclock the cpu. However, I've gone much lower than your intended budget. If you're willing to get to $700, you could either upgrade your CPU to a Q6600/E8400, or upgrade the cpu to a lesser Conroe/Allendale and upgrade the GPU to a 8800GT.