first of all, pairing TWO 8800gtx's with a 5200+ is RETARDED. one gtx will more than exceed your realistic cpu capabilities, so getting another is just waisting money.
if you want to spend that much money, just go out and buy a core2 and a whole new system. or at the least, wait until there are games out that require more than one gtx.
by the way, this post doesn't really apply that much if you have an HD monitor (like something that is AT LEAST 1920x1200), because those absurd resolutions do demand absurd amounts of horsepower.