I am not certain there's a lot you can do here. Your graphics solutions is fine (assuming both monitors are running from the card). Your motherboard will support more RAM, but that would require tossing those 512 sticks, and generally the performance increase above 2Gb will be marginal. It's not the fastest CPU around, but it's not a slouch either.
The bottleneck appears to be the motherboard which is limiting you to relatively slow DDR RAM across a slow bus. The only way to achieve significant improvement is with a new board, one that supports DDR2 or DDR3 memory, and faster CPUs. This, of course is a major upgrade, one that may require a new power supply too.
Short of that, you can buy a slightly faster CPU but with a 939...