Yes - That UPS would give you enough time to do a clean shutdown but not much longer and would not really support an upgraded of your video card or much overclocking. I dont see a runtime chart on the UPS website so I am only guessing when I say you'd have about 5 minutes with a 300w load.
And by the way, that (single) ups would not run both pc's. You would need 1 ups for each.
Under heavy load if there is a fluctuation in the current you might see a reboot IMO.
I'm using a 1.2Kva ups on a 600W system, and that goes for a kick start every time we have fluctuations.....
You'd be safer off going for a better brand like APC or something of that sort. Especially if price is not the problem and the work you do on the rig is a priority.