Deaner, I have a cheap 6500 watt back-up generator that is brushed. It runs my computers just fine. As long as the power cycles between 55-65 and voltage is between 110-130, your puter will be fine. The cheaper generator's voltage tend to fluctuate and your monitor may lighten and brighten some, but that's about the worst that will happen.

Theo is right on adding the power conditioner. They will "clean" the electrical noise and make things a LOT better on a gen-set.
I use this one available on ebay for about $120 http://www.monstercable.com/productdisplay.asp?pin=4445