APC is the big dog in the UPS world. I also like Opti-UPS (www.opti-ups.com), just bought a PS500B from them for my a/v gear because it does really nice line conditioning -- both voltage and frequency boost and buck.

VA doesn't directly relate to runtime, a 900VA UPS with four 12V 17A batteries will last for two hours, a 1000VA with two 12V 7.2A batteries lasts about 12 minutes with the same load.

There are ways of connecting the UPS to one computer and running software that will communicate over the network to shut all computers down.