UPS's range in price from $30 to thousands. Generally, more money will get you more power (and it will also be larger). But if all you want is something that will keep your computer from rebooting if the power blips, a cheap one is fine.

Read the back of the boxes. (Or the descriptions, if you're buying online.) Generally, the UPS is rated for certain systems. ("This unit will run a Pentium IV with 19" monitor for 15 minutes.") More money will get you longer times. (If you have an LCD screen, you don't need as much power.)