HDD - $64.99 - 1TB 7200 RPM HDD
http://www.newegg.com/Product/Produc...82E16822148433
Video Card - $209.99 - Geforce GTX 460 1GB 256-bit GDDR5
http://www.newegg.com/Product/Produc...82E16814127510
Processor - $159.99 - AMD Phenom II X4 965
http://www.newegg.com/Product/Produc...82E16819103727
You don't really need a 6-core processor in a budget PC. Most software isn't even optimized for 4 cores yet.
Motherboard - $109.99 - Asus M4A88T0V EVo/USB3 AM3 AMD 880G HDMI USB 3.0
http://www.newegg.com/Product/Produc...scrollFullInfo
Memory - $47.99 - A-DATA Gaming Series 4GB (2 x 2 GB) 240-Pin DDR3
http://www.newegg.com/Product/Produc...82E16820211409
PSU - $44.99 - APEX AL-D500EXP 500W
http://www.newegg.com/Product/Produc...82E16817154026
Total: 637.94
I little over your 600 limit. If you can reuse a hard drive or power supply then you are golden. My advice on a budget is don't skimp on the video card it's the most important piece in a gaming rig. Don' be tempted by intel's i3/i5/i7 line, AMD is great for budget PCs. You don't need a 6-core processor though!!!
Good luck.