I'm running a q9400 and ddr2 800 8gb... gtx 275 or gtx 650 for my main gaming station? I already own them both
The 275 will be faster by a decent margin. You could run the 650 as a PhysX card, but unless a game you play is has lots of PhysX effects (and even then), the extra power draw isn't really worth it.
judging by that the 650 and 275 should be approximately similar in performance.
So I would use the gtx 650 as it is newer, has MUCH lower power consumption and will be cooler and quieter. Also the 275 will not be able to use direct x11.
Edit... I also agree with Cons29... you have them both, try them out and see which is best