Re: intend to buy GTX 275
I'd go for the 275 since it will be a bit more 'future-proof'. I have a 275 and it's a damn fine card but I'm also running a quad core. But in all honesty I think if you were to compare both cards side by side with your system the 275 would still show a nice improvement over the 260.
DFI LP UT P35-T2R / C2Q Q6600 @3.0GHz / 8 GB GSkill F2-8500-CL5-2GBPK / EVGA GTX 275 / X-Fi Titanium Fatality Pro / Windows 7 x64