I run a 500va trippe lite (IBM branded) on a 431w p/s, 21" monitor, 3 hard drives, xp 1800@1725mhz, dvd, cdrw, etc and a boatload of fans (2 hd coolers, a 120, a 92, 3 80s, slot cooler, etc) and a cable modem. never ran it for more than 4 or 5 minutes before i shut it down, i imagine im pushing it. if you just need it to shut down your pc it should be plenty. if you want to continue playing UT you need something bigger 🙂
if it does run out of juice it will just shut down, as if the power went out.