That's a socket 775 CPU I think, which means an older motherboard. Which means newer RAM is probably not compatible, and finding older RAM might be a problem. Maybe not, but something to consider.
Also, I have the same video card, but a much better processor and more RAM. It's rare that my GPU is the bottleneck, though, if I'm willing to turn down graphical settings. This leads me to believe that you would not see a lot of improvement with a better video card; I realize a video card is by far the easiest of all upgrades to do to a system, but I think you would get very little bang for your buck. The CPU/RAM combo is most likely your issue, but changing those probably also means changing motherboards.