Nvidia GeForce GTX 460 can consume up to 248-270 full load stock clock settings.
The GTX 460 requires a +12 Volt continuous current rating of 24 Amps or greater.
Sapphire HD 7870 can consume up to 246w full load stock clock settings.
The HD 7870 requires a combined +12 Volt continuous current rating of 27 Amps or greater.
The GTX460 can require a little more wattage then the HD 7870, but also require slightly less Apms from the +12v rail.
I currently own both the GTX 460 and Corsair CX500 power supply with my processor overclocked to 4GHz.
The Corsair CX500 has 34A on the single 12V rail.