Presuming your system draws 500W and there's a power outage, a 600VA UPS will simply power off. You'll never have time to shutdown your system.http://www.apc.com/products/family/index.cfm?id=27&tab=models
Select the maximum wattage that you believe your system will draw and you'll get a list of appropriate UPS, etc. For 500W, a APC Power-Saving Back-UPS Pro 1000 will provide 6 minutes.
If you stop applications that keep the system very busy as soon as there's a power failure, then you don't need more than 2-3 minutes. Once the system is idle, you'll have lots of time to shut it down.