Well, I dont think you need above 1kva UPS for 10 mins, Indeed you have a 800w PSU but it's way more than What power your system is pulling (considering you have a HD 5570 and also that only the case is connected to UPS) - you can use
http://www.thermaltake.outervision.com/ to help you get an idea of your actual consumption
I can easily last 10 -15 mins with my system + 23" monitor on a 1kva ups
So if you were to buy a new UPS, get a better more reliable brand for a lower va (lower than 1400va u stated above) ... I mean flames out of UPS sends chills down my spine, especialy if its set on a wooden table, and your not home!