I don't want this to turn into a religious argument... but, you might do some research into APC. They have a real tendency to eat batteries due to overcharging, resulting in either battery bulging (making them damn near impossible to remove from the case) or outright rupture. I've been pretty happy with Cyberpower's offerings of late. They are also substantially less expensive, and I've been pleased with their customer service (out-of-warranty unit, simply went tango uniform one day... lights were on, no one was home... called them up - don't know why that happened, but give me the serial number and I'll ship you a brand new unit with a return label for the old unit, no charge...)
You could get away with a 1.5KVA UPS, although you won't have a ton of headroom to add other stuff. You need to size the UPS based on the spin-up current, which will be pretty high. My system (in my sig) idles right at 500 watts, and will jump up to 600-650 under intense CPU load. However, for about a minute at boot, it'll hit 1.2KW as all the drives spin up, before the BIOS posts, etc. That's what you need to size for.
As
@jgreco points out, you'll almost certainly need a dedicated circuit to run a system of this magnitude. Don't forget the cooling as well... these beasts produce quite a bit of heat.
