The fact that your power supply is 800W max doesn't mean it will actually consume that much. Most likely it will operate at no more than 400W. You would be okay with a 900W power supply. Most likely the 900 you talk about is a Back-UPS 900
or something similar. I wouldn't recommend going with any of those units for a good PC that has important work on it. The Back-UPSs don't output a sine wave and aren't very good for high end equipment. Get yourself a minimum of a Smart-UPS 1000
if you want dependable protection. The Smart-UPSs output a sine wave and offer much better power filtering and protection. If you'd like to have a little more run time, get yourself a Smart-UPS 1500
. The 1500 will definitely support the power supply and any small appliances you have to go along with it. It's a data center unit. Doesn't make sense to spend money on poor protection.