You will also need to bear in mind what your PC equipment is drawing - if you have an LCD as opposed to a CRT monitor, the LCD would draw less - if you are running a top end PC system, with all the bells and whistles at full throttle, then chances are that you'll only get 10-20 minutes from a 1500VA or 2000VA UPS anyway.
I currently have a 2x 450W PSU's running my PC (AMD 4800+ X2 CPU, 7800GS GPU, 10x HDD's) on a 1000VA LG UPS - can get approx. 20 minutes on normal use before shutdown (25-40 mins on idle, 5-15 mins on full throttle) - my LCD, ADSL modem and speakers are on a second 700VA UPS.
As mibloem mentioned, you'll definitely have to look at the sustained load capabilities - don't expect to have a PC running as normal for hours...for that, you'll need a generator with the UPS...
Cheers.