|Prices are approximate:|
Core 2 Quad Q9650 Yorkfield - $330
4GB (2 x 2GB) DDR2-1066 - $90 (brings your total to 8GB)
GTS 250 low profile video card - $115
PSU - ??
1.5TB HDD - $75
Total - $540 + power supply
Your PSU is only 220W so it will HAVE to be upgraded. Finding one to fit in your undersized case is going to be a problem.