For AMD route, you can do...
X2 3600+ Brisbane - $72
Biostar 690G - $90
http://www.newegg.com/Product/Product.aspx?Item=N82E16813138057
SuperTalent DDR667 2x1GB - $90
160GB SATA Samsung - $50
DVD Burner of your choice -$40
Total is ~ $350, giving you $150 to pick a case and power supply, and maybe a sound-card if you hate Realtek or value proper EAX support. The brisbane should overclock well to performance of stock E6300. The on-board X1250 isn't too well in gaming, but if you're willing to wait, then it should allow you at least play CS:S at 1024x768 w/o too much drop in frames.
If you want to shoot for C2D:
E4300 Retail - $120
P5B - $120
SuperTalent DDR667 2x1GB - $90
160GB SATA Samsung - $50
DVD Burner of your choice -$40
Total is ~ $420, giving you $80 to pick a case and power supply, and a low-end video card. The on-board ADI for the P5B is actually much better than any Realtek one, so unless you hearing is fine-tuned, it should be more than enough. It would be really hard to stretch the $80 left in budget for buying any decent combination of any of the remaining 3 items, so I'd suggest increasing it by $50 more. C2D will definitely blow your budget no matter how you shape it, unless you sacrifice the motherboard quality. However, the E4300 will overclock decently and far surpass anything that the Brisbane is capable of.
This is on sale for ~$50:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814102035
Should provide enough horses to do most games @ 1024x768 and lower quality @ 1280x1024.