What's the brand/model of the PSU? Well, the 460 requirements recommend a PSU of 450 watts, while the 240 is 300 watts. If it is a good PSU, it should handle both. However, I'd feel better bumping the PSU up to a 700-750 watt system to play it safe, especially if the PSU you plan to use is a bit older and has seen a lot of use.
The recommended wattages are for the entire system. Since the GT240 doesn't need a power cable, it adds a max of 75W (actual is lower). Added to the 450W recommended for a system containing a GTX460, a solid 550W PSU would be sufficient.
Antec, Corsair, Seasonic, and Enermax are among the better brands.