I have a couple of doubts regarding this as well. Is the 550VA model mentioned above a sine wave based UPS? If I require about 10 minutes of backup, how much "VA" would I need for a computer that runs on a a 500W power supply?
^^ AFAIK pure sine wave UPS starts at 750 VA in APC which is available for ~7.5K.
If your PSU is 80% efficient , it can at the most supply 400W of power, hence a 650 VA UPS will be able to supply power for approx 2-5 mins at peak load.