I would only plug in your PC to the UPS so that it keeps the load down. I run an APC SmartUPS 750VA on my system. It keeps it up for maybe 10-15 minutes or so (old batteries), but I have it set to shut down if the power is out for more than 5 minutes. If I am in game, it won't hold it very long at all. http://www.apc.com/resource/include/techspec_index.cfm?base_sku=sua750
APC does have a selector to help your choose. I am not sure how accurate it is, but it might help. http://www.apc.com/tools/ups_selector/index.cfm
The big thing is, in game with my old GTX580 I would cause the UPS to blink and if I overclocked it to much it would go into over volt mode. With my GTX680 I am fine, but it is something like 85-90% load in game. I don't believe I've lost power yet in game so I don't know, but with the 580 it just shut down. So a 500W 750VA is probably where you want to be, but they aren't cheap. I got mine for free, but had to get batteries for it. The only bad thing about my setup is the LCD is not plugged in so I can't see what is going on during a shutdown. I have it mainly for power flickers.