Obviously some games take advantage of SLI better than others. So you may get the same performance in some games, or for games that don't SLI well you might see improvements. But honestly, a GTX 470 for the most part is a side step from what you have.
A GTX 570 is faster than the GTX 470, and is probably where you'd want to start looking.
I was going to suggest the SLI GTX460 as well. You don't seem to have an issue with SLI, and that setup is as good as the GTX480/570. If a single GTX570 is less you can get it, but the SLI GTX460 is a very powerful option.
you already have a fast setup. i would at least wait for the gtx560 to be released, 2 of them may be the better option. but a better option would be to get a fast single card, then you can add another at a later date. You may also want to look at a CPU upgrade depending on what you already have.
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.
Using a dedicated card does have overhead but if your current cards are scaling well compared to a single gtx 260 and on par with a gtx 470 then you should be fine in that regard but don't expect wonders. I would hold off for a little longer unless you got that itch or a problem with one of the cards then go ahead and make the move. Personally I wouldn't get a GTX 470 or sli two gtx 460 but the GTX 570 might be a best in slot upgrade for the cost and lowered cpu load.
My system Asus p6x58d prem./ intel i7 950/ Corsair CPU cooler H50/ 6gb Corsair TR3X6G1866C8DF / two evga GTX 260 core 216 896MB DDR3 MB/ CoolerMaster 922 case/ 850w PSU Ultra X4/ WD HD sata 350 gb x 3/ Windows 7 64bit Pro/ I just did a 3D mark vantage trial benchmark score was P22253.64
What do you think of the benchmark number?