I've been using uninterruptable power supplies (UPSes) for a long time. I've got 'em on my Mac Pro, on my two NAS boxes, and on pretty much every other computer in my place. But yesterday, the UPS on my Mac Pro went berserk and cut off power to my system. It happened again today. When I moved, someone else actually set up my computer(s). Well, that's not entirely true, but someone did actually place my computers in their respective locations and plug some of the stuff in. I didn't bother to check to make sure that the right UPSes were plugged into the right devices... which is why I had two hard power-offs in two days.
It turns out that a 350VA UPS was attached to my Mac Pro instead of the 1400VA UPS (!). People on the forums recommend 1500VA for use with a Mac Pro (here is the one I'd get). The big desktop machine clearly draws far more power than a 350VA system can handle, and when my CPUs cranked up while doing some transcoding, the poor 350VA beeped its little cry of death and cut power to my system. I must be getting old and forgetful because it happened twice before I noticed what was going on.
What's funny is that I was on the phone with Sea Shepherd advising them on website stuff, and my computer went down during the call. I'm sure that inspired confidence. ;)
So the lesson: always set up your own computer. Don't trust anyone else!
UPDATE: I will be running dual 30" monitors as well, which draw approximately 177W maximum, each. I ran this through a Watt to VA calculator, which suggested a 650VA UPS to drive the two of them. I'll probably end up getting a 750VA model.