Then you have an upgrade path to PCI express, and Core 2 Duo when you need
Depends what you expect.
A 7600 GT will give you a big boost in many games over the 6600 GT... and might even pet you play many games at 1280x1024. However, if you plan to play next-gen titles at high res and full detail, think again.
A $180-ish X1950 PRO would make more of a difference, but you might be better off investing in a PCIe Core2 Duo system at this point...