I'm going to guess that you have the system and the money to buy those cards. With that said, go with either the 6850 or the 5850. There both pretty powerful cards and can at-least play any game on medium settings.
Al-though i don't know how they act in crossfire. I think you've heard the horrors of micro-stutter. I'm going to try it out this upcoming Chrismas. (When i have a little more money in the pocket). ;O
I do have the system, and I have money to spare for a GPU. I figure CF would still be a ways away for me. I'm not too familiar with the micro-stutter problem. Is that an issue with specific cards, CF in general or this generation of cards?
So even at $80, HD 6790 is not a smart move, huh?