Re: PSU Question.
I know you also have to pay attention to the number of +12V rails, but I'm not 100% sure on how that works.
Along with the obvious thing wattage, there is amps like Joga mentioned, and another thing that I think would be pertinent to you would be the number of PCIE connectors. While it is possible to use a 2x molex to 6 pin PCIE adaptor, I feel its better to just use the "real thing". I'm not sure what the connectors will be like on the 5870 x2, but I assume it will be something similar to the 4870 x2 or the gtx 295, where the card will require either two 6 pins or one 6 and one 8, some might even require more. For example I know EVGA's new classified GTX 285 requires 3x 6 pins. What you would want to do before you buy is find out the number of PCIE connectors(pay attention to # of 6 pin and 8 pin) that two 5870 x2's would require, then make sure that the power supply you will purchase has that many connectors.
On a side note, I would think that you would need a decent 750 watt - 850 watt for a single 5870 x2, and then maybe close to a 1200 watt psu for two 5870 x2's in quadfire. Unfortunately a 1200 watt psu is going to be VERY expensive. It might be more economical to buy two seperate psu's(purchase a case that can hold two like the HAF 932 for example) and use one for your system and the other just for the cards. Maybe something like a 500 watt for your system(MB, CPU, HD's, fans, etc.) and then a 600-700 watt just for the graphics cards. Again, I'm not 100% sure on what your power requirements will be, so those wattages are just rough estimates, and again it will come down to other things like amps too.
Hope this helps,
Foxconn FlamingBlade, Intel Core i7 920 OC @ 3.63 GHz w/ Asetek 240mm LCLC, 3 x 2GB Mushkin Redline DDR3 1600 @ 1734 6-7-6-18, 2x 150 GB VelociRaptor raid 0, WD Caviar 1 Terabyte, 2x(sli) EVGA GTX 260 core 216 OC'ed, Creative Sound Blaster X-Fi titanium, Wireless N, Vista Home Premium 64-bit, 850 watt Corsair psu, Blu-Ray player, DVD/CD player/burner, Acer H233H, Samsung TOC T220HD, 32" Vizio 1080p, HAF 932 case, Bose Companion 3 multimedia system