Welcome to the forum! :-)

For that board and CPU range, I suggest an Nvidia 9800 GT with 512 MB RAM such as this MSI N9800GT 512M OC GeForce 9800 GT 512MB.

I have an nForce 4 based system with a Socket 939 FX-60 processor clocked at 2.8 GHz. It is nearly equivalent to an Athlon 64 X2 5600+ and does reasonably well with my Nvidia 8800 GT 512 MB card which is nearly identical in performance to the 9800 GT.

Be sure to uninstall the previous drivers when you swap cards.

Be advised the Nvidia 9800 GT requires a six-pin auxiliary power connector. You should verify your power supply has such a connector. Speaking of which, what is the brand and model/power rating of your power supply?