Get an apc smart-ups which produces a reasonable sine wave, not a 'modified sine wave'.
A so called modified sine wave is really a square wave with some 0 volt output to make it a little nicer. See
http://www.weasel.com/comp_ups.html#upsover
for drawings of the waveforms.
I happen to be selling a few smart-ups's as well as a 900va back-ups.
The smart-ups's also have better monitoring options, with ethernet monitoring options, unlike the back-ups.