Knowing that there are probably a few bits of hardware that you didn't list, you probably need a PSU of around 500wt. That would allow for a mild overclock as well. An Antec Earthwatts 500wt would do nicely. By the way, if you want more exact results for estimating the size of a PSU, go to:
Because of the huge power draw at load for some of the new graphic cards, i'd go much higher with a PSU, especially where Xfire or SLI may want to be used. HD4870 est. 157 watts, HD4870X2 est.250w 260GTX est. 182w, 280 GTX est. 236 watts. Since you're building this at the end of summer you'll have a full choice in which GPU to use. The X45 motherboards are also going to be out soon, which may change your mobo choice. For PSU i'd be thinking a 750w just to give you some headroom......... bigger if you were going with 2 GTX280s. http://www.atomicmpc.com.au/forums.asp?s=2&c=7&t=9354
For two 8800GTS cards, the HX1000 is way overkill. For two 9800GX2 cards, it's just fine according to that certification list. For two GTX 280 cards it will also be enough, because those cards will consume a little less than the 9800GX2 AFAIK.
The rails don't really matter, as long as combined they have enough juice. Some people say one strong rail is best, and some say multiple smaller rails are best. For example the PC Power & Cooling Silencer PSUs have one rail, and the Enermax Galaxy series have multiple rails. Both are excellent anyway.
Problem with the Thermaltakes is that some of their PSUs get great reviews and ratings and other get poor reviews. You need to check exactly which PSU you'd be getting and do the research. PC power and cooling and Corsair PSUs have all gotten good or better ratings throughout their lineup. http://www.hardwaresecrets.com/article/181 A good general tutorial about PSUs.