Yes, wait for another 4-6 months and buy a new video card when Nvidia and AMD release their new products. Until then, don't upgrade anything, not enough difference in performance in games, at list i would not upgrade. Games should still run fine on that PC.
An improvement would be to buy a CPU cooler and overclock your CPU.
If you really want to upgrade and go Nvidia way 1 GTX580 or if you want better performance buy 2 GTX560ti and put them in SLI. That should be around 500$.
AMD way, 2x 6950 will hurt everything with a single GPU, even gtx560ti in SLI. But it's not Nvidia(in case you prefer that company).