I believe its backwards. Radeons usually use less power over the last two generations. The 460 requires 2x6 power connectors while the 6850 only requires 1x6, which means they use less power.
As far as heat is concerned, depends on which brand you get as to which cooler is on there. If a card needs more power, it will run hotter, so the 6850 should run cooler as well.
Oh, ok well thanks for the info, I'll check into the Radeons a bit more