This is for a quad 2.66 - an octo 3.0 would probably be about 550 or 600 watts at peak load.
http://discussions.apple.com/thread.jspa?messageID=5363179
You could use APC SMART-UPS 1000VA or the RS1500VA. There are lots of threads on UPS needs and demands.
The more RAM and drives, and PCIe cards, the higher.
Power Usage of Mac Pro It requires 410 watts to start up, idles at 300 watts, and peaks at 430 watts when running Doom 3, the most demanding power wise of the ten apps we tried.
The Mac Pro had 16GB of RAM, four internal hard drives, and a Radeon X1900 XT. We're confident the Mac Pro's 980 watt power supply can handle just about everything you can do to upgrade or expand a Mac Pro.
Mac Pro: Power consumption and thermal output (BTU) information
Update:
http://docs.info.apple.com/article.html?artnum=304378
Mac Pro (Late 2006) Quad Core 2.66 Ghz Mac Pro
Idle and max consumption:
171 W 445 W
854 BTU/hr 1,519 BTU/hr
Notes
*Power consumption data (Watts) is measured from the wall power source and includes all power supply and system losses.
Additional correction is not needed.
*"CPU Max" is defined as running a compute-intensive test application that maximizes processor usage and therefore power consumption.
*These numbers reflect a 23° C (73.4° F) ambient running environment. Increased ambient temperatures will require faster fan speeds which will increase power consumption.
At 35° C (95° F), 50 W should be added to reflect increased power consumption.
*1000W PSU do not deliver more than 500-600W effectively