At the resolution you're running, I'd get the GTX 275. All the reviews I've seen suggest it's worth the additional cost over a GTX 260. Here's a few articles, if you want to take a look...
http://www.overclockersclub.com/reviews/nvidia_gtx_275/
http://hothardware.com/Articles/NVIDIA-GeForce-GTX-275-Unveiled/?page=9
According to overclockersclub, it won't be widely available until the 14th of this month. They're all sold out at the egg.