kcobra, Would you be so kind as to tell me what PSU's you'd go with if you were building this rig? That would be great.
Personally I'd go for an 850w XFX PSU to power the MB, risers and one or two of the 280x's. Then power the remaining 280x's with 12v server power supplies off ebay. You can get them for dirt cheap. I just picked up two 700w HP units for a total of $15 shipped. I will use one of these in my 270 x 6 build this week. A popular power supply is an HP 900w unit for ~$20 shipped:
http://www.ebay.com/sch/i.html?_sacat=0&_from=R40&LH_BIN=1&_nkw=hp+1200w+power+supply&_sop=15Main thing to worry about when choosing a server PSU is to make sure people have figured out how to turn them on. Radio controlled plane guys have been using 12v server power supplies for years, pulling major wattage from them.