I am focusing on it, but you're not trusting me. Actually, you're not appreciating what I'm saying.
I'm sorry, but it looks like you only came here to have someone tell you that you should do what you want to do (I know that you are just asking a simple question, but I like it when someone shows me that I can spend less money). It also looks like if anyone agrees with me, then you'll just tell them to go away too.
Seriously: what are the chances that you'll make that big of an upgrade? To power your system with even two 6970s in it, you will only need a quality 650, maybe 700W at the most. So think about what you can buy with the money you'll save by buying the X-650 instead of the HX850 or X-850.
Don't let the wattage rating of these PSU's fool you. When AMD recommends 650W for two 6970's in Crossfire, they're including PSUs that have a "Peak" rating of 650W. But the power supplies we are looking at have a "continuous" rating of 650W.
But y'know what, I apologize for being interested in enabling you to spend less money. So here's my reply to the original question:
The X-850 is a superior unit in quality, has better efficiency (when the load isn't less than 50% on it like I mentioned before), and it's fully modular. But the HX850 is fine too. However, both are technically a waste of money unless you are absolutely 100% sure that you'll replace the two 6970s with something that requires much more power. Or, if you decide to add a third 6970. But you'd have to be absolutely 100% sure that you'll do it.
I might go for 750w, but I won't go for 650w.