For sizing, use the APC calculator (link above).
Don't forget:
Know your mains supply. 100VAC/50Hz is different thatn 120VAC/60Hz. And you will never get either one from municipal power supply. In 200/240 or greater voltages, you should have your mains tested to ensure that it falls within spec for the UPS. Questionable 100/120 should also be tested.
Know your local codes. In some jurisdictions, you can't fully load your racks without engineering plans. Also, some inspectors may want fire/smoke monitoring. If you don't own the building, your landlord may want to meter your electrical usage needs above 2-2.5 W/sq.ft.
Buy the network card for the UPS. You'll need it (and the softwareinstalled on the servers) to perform orderly shutdowns on multipledevices.
http://www.apc.com/products/family/index.cfm?id=98
Leave room for growth, but not too much. 50-75% loaded is more efficient than 25% loaded.
If you want more runtime, buy the extended runtime model with capacity for extra batteries. It is more expensive than the standard models, but more efficient that overbuying Watts / Volt-Amps. You could run all your servers off two 3,000VA units. Which brings us to...
When using smaller UPS units, buy two. Your servers should have redundant power supplies. Would be a shame to plug them both into the same UPS on the same electrical circuit. With two UPS units, you can power one down and remove one for maintenance/replacement without taking the servers down. Larger UPS systems will have multiple power input options, but the smaller ones have a single cord or hardwired to a single circuit.
