I had the GTX 260 c216 in my PC for a week. It gave me artifacts in certain games and locked up in crysis so I returned it since it was obviously defective when it reached a certain temp. Got a 4870 instead. But not before I did some testing. (see below)
I have an e6400 (clocked to 3.0GHz). And honestly the 260 was NOT a vast improvement over my 8800GTS/512 (OCed 750/1000). It did a little better, but not what i expected. We tested it in my buddies new i7 build and it ran like a CRAZY WILD BANSHEE. I think the new GTX's really rely on the CPU alot, so unless you have a quad core clocked high or an i7 you don't get the full benefit.
With a slower CPU I would recommend the 9800GTX+ or a 4850/4870 card. They seem to do better with older CPU's. My 4870 works really well and runs great. I really need to upgrade to Vista though to get the DX10 benefit. The 9800GTX+ seems to do really well.