As usual people are adding their random 2p without giving you a proper answers...
460w will suffice on the basis your system is not already using most of the 460w i.e too many HDD's, FAN's, Opticle Drives ect ect...
The HD6850 is recommended at 500w by AMD and they base this on the average machines powers needs.
So that you can make your own accurate answer the facts are 75w is taken from the PCI-E lane MAX and on most boards and psu's combined you wont get more than 75w from a 6 pin connector. +150w
Use a simple power application to determine how much power your system draws from your 460w on full load before you go ahead.
Good luck