Don't know if anything makes sense at that price range. You have enough RAM, and as long as the video card is 1GB, it's plenty to handle anything today. You could replace the CPU with a Q9550 or a Q9400, but I don't know if that's worth $200.
Although despite what people say about LGA775 CPUs being last-gen, I still think the high-end quad cores in a DDR3 motherboard will remain good gaming machines for the next 2-3 years. It really depends on how soon you want to build a new i-series machine, and how difficult it will be to scrape together the money for one.