The HX is basically the same thing as the TX but the HX has a little better efficiency, has modular cables and build with better parts. Not to say that the TX is built with bad parts, because it is not. Both are GREAT PSU's but the HX has better cable management and slightly better features.
Well, here's the specs for a 5970 vid card... Since you are going with a quality brand like Corsair, i would say that 850 is enough to power 2 of them no problem...
650 Watt or greater power supply one 75W 6-pin and one 150W 8-pin PCI Express power connectors recommended (850 Watt with two 75W 6-pin and two 150W 8-pin connectors for ATI CrossFireX technology in dual mode)
The 850W recommendation for a pair of HD5970 GPUs supposedly takes into account that some PSUs are crappy (and not really good for 850W), so an 850W Corsair should have no trouble at all with two of them.
theholylancer has a point, so it's a case of planning ahead a little.
Work out what you're likely to add to the system in the future and go from there, however hard drives and high CFM fans don't add too much to system draw, so if you have enough headroom with a Corsair 850W then you should be fine.
You could always go middle ground and get a Corsair TX950W if all the bugs with new Asus motherboards have been worked out (what was the deal with that anyway?)
LePhuronn - Asus made a design change in a couple of their newest motherboards last year that triggered a really weird compatability issue with the TX950. Corsair and Asus had to work together to figure it out. The fix required a design change in the TX950. Corsair made the changes. I think it is the first time anything like that has ever happened. The typical compatibility issue usually involves memory.