I agree, APC is the way to go - best UPS units out there. Checkout http://www.uninterruptiblesolutions.com
. They sell refurbished APC UPS units which are a lot cheaper than buying a brand new one so you can get a better one for less. For your price range, you can get something like a Smart-UPS 700 or 1000 which will give you a good 30 mins to 45 mins to shutdown when the power fails and also gives you a really good surge protector. They include the cables with all their units over 700VA so you can download PowerChute from APC's website to automatically shutdown your comptuer if you aren't around (or you can use Windows XP's built in shutdown system from the control panel -> power options -> UPS tab)
One other thing to keep in mind -- the batteries only last about 3 years and have to be replaced for the UPS to do you any good. I always replace mine after 2 1/2 years to make sure I've always got good protection.