I'm looking to upgrade my computer for not too large of a cost.
My current setup is:

MOBO: MSI K8N SLI-F
RAM: 4x 1GB DDR 400
CPU: AMD Athlon x64 II
OS: Windows 7 64 bit Ultimate
GPU: EVGA Nvidia Geforce 8600 GTS
PSU: Random 510 Watt


I'm not sure what else I should list for my specs, im not at home at the moment and I can't remember much more than that.

Any cheap recommendations that would improve my gaming performance?