First of all, you can't put a 2gb 6850 with a 1gb one. And by that I mean it's pointless as only 1gb will be used. Second, I really think you're better off with only one GPU. 7850 is a fine choice.
sorry bout that, i mistyped my original post, i meant a 2gb with a 2gb card. Would the 7850 still be a better choice?