Depends on the clocks you run and how far your cpu can push the 480s, generally I think 460s are about 10-20% faster than a single 480, so possibly around 80% improvement over SLI 460s best case I'd say.
You'll see an increase on a magnitude of roughly double what you see now. Your minimum frames will take a hefty jump to the point where your current average will probably be your minimum FPS with the 480s.
At this point in time, probably not a difference worth going after...460 SLI are more than good enough to max those games out at just 1080p afaik. IMO if you had two of those screens you might see a difference.
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