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.