It depends on the rest of your PC, if you have a socket 939 or 775 CPU then I would replace the motherboard to PCI express, see www.asrock.com for details...
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...
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