Sorry, but I'm not going to crawl the net for power consumption data of your 5870's now - if you can provide the numbers, I could help you calculate it.
The maximum Wattage you can hang onto one rail is, as said above Volt x Ampere.
Since the rails are all at 12 Volts, the four 20 Ampere rails are able to deliver 12 x 20 = 240 Watts each and the two 35 Ampere ones will be able to deliver 12 x 35 = 420 Watts each.
As pro-tip: Don't be a cheap-ass when it comes to PSUs - they are the most important and most udnerrated part of a PC, especially since you are planning a mining rig, for which stability and reliability is crucial.
The PSUs of well-known companies don't cost 200$ more for the sake of it, it's about the PSUs delivering constant 12V, even under heavy load, it's about the PSUs not frying your Mainboard and GFX-cards when there's an electricity hick-up, it's about the PSUs not starting to burn (!) like some of the made in china ones do - I guess you get the point, unless you want to risk your house being burnt down because you wanted to save 150 bucks.
Although I don't have a clue what's recommended in the US market, since I am from Europe, I can certainly say that I've never heard super-flower before. The fact that it's one of the most cheap ones, as you are implying, should get you a bit worried to be honest.
Recommended PSU-manufacturers for Europe are: Corsair, Enermax, Seasonic, Tagan, Cougar.
Also note that your PSU should not run above 85% of it's maximum for 24/7.
Highest efficiency is at 50% by the way.