You'll see decent increase in performance. And not to mention, a single card solution would be a great advantage anyway. Especially with the GTX 460, it uses more or less the same amount of power as one 8800GT does, runs plentiful cooler and it supports DX11.
As for the CPU, bottleneck (there goes that word again) might be prevalent in some games. But since it's rated at 2.5GHz at stock clocks, hopefully you can overclock it to 3.0GHz. You won't have to worry about "bottlenecking" then.
2 8800GTS 512s should handle that game fine on your resolution,there might be a driver problem or bad optimization which gives you low FPS.
Anyway, a GTX 460 won't give you a huge boost in performance,because you play at a low resolution,if you wan to upgrade,then consider getting a new monitor first then add a more powerful VGA.