Getting two 570's would be ideal, but I can't afford it outright, and I'm not much of an upgrader. I tend to keep my PCs as-is for about 5-6 years, then just get a new one.
Looks like a single 580 (which is a faster card then the 570) only nets an 18% improvement in FPS. But dual 480's gets you a 70% increase in FPS.
But you're right, if the 460's don't cut it, I'm SOL except for overclocking...
The single GTX 580 is about 18% faster than the GTX 480, but if you go from a single GTX 480 to dual GTX 480s in SLI you will be gaining not only more comfortable framerate at about 70% scaling. The situation from a single to dual GTX 580s is pretty much similar with about 59% improvement in the framerate when using two cards in SLI, and the GTX 480 SLI versus GTX 580 SLI provides about 10% difference in performance.