UPS power requirements for server

Disclaimer: I am NOT a power engineer. Take the following as a set of suggestions, not as gospel answer for your particular situation.

Power factor
It's important to be aware of the difference between Watts (W) and Volt-Amps (VA). In a regular DC circuit, it's true that W = VA. But the process of converting sine-wave AC power to straight DC power isn't perfect, so some of the current is wasted. This is called power factor, and is different from the efficiency measurement.

Computer power consumption is typically measured in watts, whereas UPS devices are measured in terms of the volt-amps it can deliver. A power supply with high power factor (0.99 or so) will run much longer on a UPS than a PSU with lower pf (0.6) would. Most good power supplies tend towards a power factor 0.99 so it shouldn't be a huge issue, but you do want to be aware of it.

Spin up
Harddrives consume a lot more power when they are spinning up than when they are just working or idling. This can go as high as 25W. If you aren't using some kind of staggered spin-up system, you should be allocating closer to 400W for the drives, just so you can handle startup.

RAM
Is the ram registered or fully buffered? If so, it's going to consume more power than a standard DIMM. An FB-DIMM consumes something like 10W, which triples your memory estimate.

Fans
Don't discount fans too much. I have a few Scythe Ultra Kaze which each draw as much as a harddrive. Obviously, this matters more if you have a lot of fans.

Conclusion
These modifications put us somewhere around 900W. Assuming your power supplies are properly sized, you can expect to get around 80% efficiency, for about 1100W. Add in power factor and the VA is a little bit higher than that. Since you want to have at least 25% buffer, a 1500VA UPS would probably be just sufficient for your needs. A 2000VA would be a more comfortable solution. 3000VA might be overkill.

You can use a calculator like the APC UPS Selector to figure out what kind of UPS you need. I would use the estimated maximum draw of all the components and add a bit of a safety margin.

Note that the power capacity of the UPS doesn't really have much to do with how long your uptime is, that's determined by the size of the battery. A 2200 VA UPS will only be able to provide power to a limited number of machines no matter what you do, but the runtime can be extended through external batteries. Of course, the runtime will also depend on the load.

A 1500 VA UPS should be more than enough to keep this system up, but the exact model you purchase will depend on how large of a better you need given your desired runtime.