I think it is ALL personal preference on brand. You can always overclock the cards yourself pretty easily. I am partial to ASUS because I've dealt with their tech support many times and am familiar with it. However, there are alot of good brands as long as they have a good cooler on them.
The 6850 outperforms the GTX460 1GB in more than not, but not by much. As far as overclocking, they both overclock very well so it kind of comes down to preference. I am sorta partial to the GTX460 though, you can get some really good deals on them as they have been out a while.
depends on the games you plan to play, some games prefer ati card some choose nvidia. there isn't any losers on your list though, i'd say the 6850 if you plan to overclock but really even then its kind of a tossup. does your mobo support xfire or sli for upgrading donw the road... if it supports one and not the other that'd be my choice.. but if it supports niether then tell us the games ya wanna play we'll look at the ebnches on said games and be ablke to advise you better