Athlon II x2 240 (OC if you can) about $65
Radeon 5670 about $100
cheap asus or gigabyte AM2+ motherboard about $70
2GB DDR2 ram about $40
cheap case about $30
400W OCZ PSU about $40
320GB WD HDD about $50
That's as cheap as I would go if I were building a new computer. The 5670 is low powered so good on the PSU and it outperforms the radeon 3870, which can play cod6 on high settings (not extra) with AA off and always above 30fps.