With 600$-800$ you can easily double your performance.
Gas prices might be going up, but computer prices are relatively cheap now.
Make sure you get a pci-express graphic card. I recommend geforce because ati has ALOT of compatibility issues.
Get ddr2 ram, at least 1.5gigs.
And get at least a 3.0ghz processor.
http://www.ibuypower.com/ibp/store/c...r.aspx?mid=224
check that out.