IMHO, his current PC SHOULD be capable of running Mario Kart Wii fine most of the time. If the built-in HD Graphics on my G3258 (which is much weaker than the HD Graphics 4400 on your i3) could stay at 60fps most of the time at native resolution, you should be able to keep it pretty solid. Granted, the IGP IS the limiting factor here if you want higher resolutions.
A GTX 650 Ti like OP is looking at will be more than enough to get at least 2x in most games.
A GTX 650 Ti like OP is looking at will be more than enough to get at least 2x in most games.