here is some reading about the different types of ups:
http://en.wikipedia.org/wiki/Uninterrupt...wer_supply
usually a cheep line interactive would be sufficient, how ever it sounds, you have a big server with many disks.
I would check your power requirements first, the usv must support enough Watt to your server.
I have the PowerWalker Line Interactive 2000VI ( 2000VA or 1200W). it has 2 battery packs 12V / 9Ah, what means 216VAh.
So if you have a device, that consumes approxymately 120W AC (~200VA), it could run about 1 hour if the battery was full charged - until it is discharged completely.
So there is a chance in real world to survive about 45 Minutes (deep discharge protection, battery wearing over time), before the UPS turns off - as a rule of thumb.
Online it uses 10% of the total capacity (120W out of 1200W possible)- so the fan is off and quiet and the device is almost cold.
http://powerwalker.com/line-interactive.asp
If you take the weakest (450VA / 240W, Battery capacity 12V / 4.5Ah), it means 54VAh or approx. 30Wh, and you run 120W, you will end up 15 Minutes best case.
I would think 10 Minutes is more realistic, and you will have a heavy load on it (50% - 120W out of 240W) , so it could get warm and noisy...