At those resolutions with that processor bottleneck, you'll get about 30-35 fps. I have a single GTX 560 ti 2gb, and I get about 45 fps with an overclocked core i5 2500k (4.2 GHz.) I suggest I GTX 570 or wait for Kepler. Here's a list of Kepler cards, predicted prices, performance estimates, etc: http://www.tomshardware.com/news/Nvidia-Kepler-GPU-GeForce-600-Series,14642.html
Hope that helped.