Fitter,
I agree with the above posts. I moved from 2x evga 260core216's superclocked to 2 570SC's.
2 260core216 SC (not stock) models are right on par with a 470SC model. But keep in mind that a 470 is basically double the performance of 1 260, and games you play that don't support sli will really be hurt by your current setup. If you really want double the performance or better for a reasonable price, try 2 470s. I have seen them as low as $250. So for $500, you would have more than enough gaming performance.
If you just looking for a place to start, consider this.
Buy either a 470/570 single card, and re-use one of your 260s for a dedicated physics card. By offloading your physics workload to your 260, you free-up your new card for only 3D rendering.
If you like the performance your getting, then you can either sli them, or just leave it like that until the 600/700/800 series cards.
One question that I didn't see asked is, what resolution are you running at? If you want awesome FPS at 2500+ res or running 3D, I would only consider 470s/570s in sli or better.