HDD/PSU: Samsung Spinpoint F3 500 GB and OCZ ModXStream 700W
$95 after rebate
Optical: Cheapest SATA DVD burner $25
RAM: G.Skill Ripjaws 2x2 GB 1600 mhz CAS Latency 7 $115
OS: Windows 7 $105 (didn't list as not needed...)
All of that totals: $340. That leaves $260 for the CPU and mobo. That means the i5-750 is out of range. In that case, I would go with the Phenom II X4 955 and Gigabyte GA-MA790GPT-UD3H for $256 after rebate.
If Windows 7 is not needed, either of those combos would be good. To be honest, I'm not seeing much of a difference, so get the cheaper one.