the 5850 will be better, on power too. but i would get a 6 series if possible.
http://tpucdn.com/reviews/Powercolor...power_peak.gif
now if you fold or use cuda? the 460 will be a better buy.
HIS Radeon HD 6850 1GB GDDR5 PCI-E w/ Dual DVI, HDMI, DisplayPort at Memory Express Computers Gigabyte Radeon HD 6850 1GB GDDR5 PCI-E w/ Dual DVI, HDMI, DisplayPort at Memory Express Computers
although for your current resolution i wouldnt touch any of these, because their value will
go down by the time you decide to get a better monitor.