I'd stick with Nvidia for CUDA support in your case so a GTX 570 would be your best choice. It's not quite the best but it's close to it. There is the GTX 580 which is better but it's really not worth the extra cost IMO. About 15% faster while costing 50% more.
Looking at newegg this is probably the card I would choose at the moment:http://www.newegg.com/Product/Product.aspx?Item=N82E16814130684
It should be well over twice as fast as your GTX 260.