UPS's are rated in voltamps (VA) or kilovoltamps (KVA), an indication of power for a certain amount of time versus draw.
For most desktop systems, 450-900 VA is more than sufficient to save your work, shut down, and have no worries. You should expect 30-45 minutes minimum powering a tower PC and monitor.
Any APC unit is outstanding, I highly recommend them (
http://www.apc.com/products/category.cfm?id=13&subid=5). I specifically recommend against Monster, as they trade their name for price, and none - and I mean NONE - of their products is that superior or worth nearly the prices they charge. (I expect a cease-and-desist notice from Monster's attorneys anytime now, they are litigious bastards...)
Oh, remember to replace the batteries every 18-24 months, and don't run a laser printer through your UPS!