dont forget to add the wattage of all your devices that you want to be up. 500VA = 300W. if youre running a full load on your computer power supply, 500VA wont be sufficient to cover both the monitor and the computer, much less anything else you want to run.
if youre running a power hungry comp, youll want a 650VA or more. and any UPS that costs under $100 is really a pos. well, except for those of you living in so cal, where the power never goes out
ive got a 400VA ups that i bought over 5 years ago for $150. it still works quite well. and it gets used almost daily. ( the joys of ohio weather) you get what you pay for.