Your system doesn't draw 500watts. That's just the PSU rating. It probably runs anywhere from the 150 watts that someone else suggested to a bit over 200watts on average. For 5 minute of run time, provided you don't put too much non-essential stuff on it, you won't need that big of a UPS.
I vote for APC... I had an exceptional support experience with them when my 1500 failed, 11 months into the 12 month warranty. They sent me an advance replacement, Brand New.... and of course with new batteries. It was so easy.