I use a Cyberpower 900W unit in a mid-high end pc and get a full 40 min back-up time.
I'm very happy with it and recommend Cyberpower.
I agree with mosox that a 650VA would be fine for your application but looking at current pricing you can get a better unit for a good price.
The back-up times listed are very conservative imo.
I'll bet you'll get at least 20 min with one of these.http://www.newegg.com/Product/Productcompare.aspx?Submit=ENE&N=100007932%2050001821%20600013484&IsNodeId=1&bop=And&CompareItemList=72%7C42%2D102%2D085%5E42%2D102%2D085%2DTS%2C42%2D102%2D082%5E42%2D102%2D082%2DS01