Agreed, Getting a better graphics card will see your numbers jump. 4 Gigs of ram is okay though. The main problem I see with your setup, is your CPU. But then again, changing your CPU + MOBO might be out of the question at this point.
So the easy route would be to get something like 2 6870s, or two 560 Tis. The TIs would give you better performance, but would also cost a bit more. With any money left, get a PSU that has enough cables for crossfire/sli Support. Also, if you're going SLI/Crossfire, check to see if your MOBO has enough PCIe slots.