Well after looking at your rig, there's actually a lot you can do with $400. Spend $60 on a good Power Supply. Spend $90 on a decent board. Spend $90 on a Phenom 2X. Spend $45 on 4 GB (2x2GB DDR3 1333) of ram. Spend $90 on a 1GB 5670 GFX card. You can even pay for shipping.
You'll have a decent set up that will be way ahead of what you have now. It might not hit 80 FPS, but the difference will be night and day.