Whats your budget for an upgrade? You dont need a super strong card, a 5770 would do quite well, but something like a GTX 460 would fare even better and let you max most things out. Your CPU isnt going to be providing a relevant bottleneck to any card so dont worry about that, just dont spend a ton on a card like a GTX 570 that isnt going to be able to stretch its legs at your resolution.