The 460 is the better card here, but not by much. It has slightly higher performance and better SLI scaling. 768MB isnt going to hold you back much now. Just turn down the MSAA.
The HD5830 isn't bad though, power consumption is slightly lower, but is a longer and a slightly warmer card. But unless you want eyefinity, there's not much point in one as the 460s the same price.