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.
What 750W PSU do you have? Most 500W units are fine for a single graphics card, so unless you got a super bad 750W unit you have more than enough power available.
A GTX 460 and a 5770 both need two slots next to each other in the case, but only need one PCI-E 16x slot. What case do you have? Looking at your motherboard, unless you have something in both PCI-E 1x slots you will have enough space.