Hardware is cheap these days. IMO, if your gonna spend over 200$, do a new build. Your buying outdated ram and an outdated GPU interface that you wont be able to ever use again.
Heres a price estimate for you:
Reuse your optical drive, HDD, Case, and maybe PSU if you can.
CPU: Intel Pentium E2160 68.00
Mobo: GIGABYTE GA-P35 97.00
RAM: CORSAIR XMS2 2GB 40.00
GPU: 9600GT 512MB 150.00
Total: 355$
A new PSU would be 40-50$, and if you dont have a spare OS around, another 100$. Which would make my idea less realistic, if budget is a big concern.
But as you know, the performance gains would outweigh the price difference.