I was in a similar conundrum way back when I had my X2-4400 system, and I finally replaced my 7900GS with my current 8800GTS G92. Yes, I was getting bottlenecked by the CPU, but at least I did not have to get another card when I changed to the Q6600. I'd say go with that 260, like that you will have a very usable card when you upgrade to whatever other mobo/cpu in the future.
Just for the record since we are also talking about PSUs, I used a Seasonic S12 550w to power that system with the G92, but that is one of the most expensive 500w-ish PSUs out there.