I would usually pay a premium for APC. I've had a few different UPS's and APC's are the only ones that let you turn off that annoying beep (and only some APC's at that). Also, APC's last a long time for UPS's.. My APC that's 3 years old is still running like a champ and recharges to 100%.
My belkin 800VA runs my main box (4 hard drives) and my linux box (one hard drive) and shows it at around 60% load. It will give me about 5 minutes total. (no monitors)
For both computers and the LCD screen, I would say you'll need at LEAST 1000VA and I'd recommend 1500VA.
The advertised uptimes are supposed to show how long they would run at 50% capacity. However, they're usually highly exaggerated. Expect about 1/2 as long as the advertised uptime at 50% capacity. And, power drain is not linear. This means that a UPS running at 50% might last 20 minutes while the same UPS running at 80% might last 4 minutes.