I hate to disagree but I just had to replace a Corsair HX1000W because I did not do a power calculation beforehand and figured it would be more than enough to handle an Asus P6T6 WS Revolution, Core i7 Extreme, 2 HDDs, a single DVD drive, 6 case fans (nothing fancy or very power consuming (5 x 180mm, 1 x 220mm), AND three (3) BFG Geforce OC 275 GTX's. I use a modified Promethia Mach II phase change unit for cooling which has its own power supply.
When I ran a Corsair HX1000W the MB would light up for a second or two and quit, period. The Corsair was and is running fine as I have tested it at length with an Antec PSU meter with no variations or spikes. I figured it had to either be a courrupt BIOS or a PSU issue and ran a power need calculation at: http://www.antec.outervision.com/ (I have a personal preference for this calculator over Nvidia's simply because it is more comprehensive) and learned that I required a minimum PSU of 1,250w. I had a 1,600w Ultra X3 (yes not 1,500 and I have no idea why they changed) modular I had been using when I was running 4870 X2's (which I regret giving up) but switched as it only has two PCI-E 6 pin plugs and liked the Corsair configuration. Bottom line when I installed the X3 the board began to post.
I took the liberty of running the power requirement for three 260's through Nvidia Sli Zone [http://www.slizone.com/object/slizone_build_psu.html] which basically puts your need at at least 1,200w. The majority of PSU's recommended are 1,200w (even above). While there is a 950w listed as well as a 1,000w or two, I believe, but am no electrical expert, that they are likely unique as the ones I have any knowledge of listed by Nvidia in the lower range are unique designs. For instance they list the Enermax Revolution 85+ 950w PSU as acceptable for 3-way 260's; however all of this series of Enermax cards output more than rated. For instance the 950w Revolution will output 1,140watts (enough difference to run a card most of the time).
Long story short check with the experts and my rule of thumb is to add a few hundred watts as I don't know about you but when a good 3-way cooling solution comes around I will be the first in line. GOOD LUCK