This was originally posted on APC forums on 12/12/2008
The UPS you choose will depend on multiple factors, including the amount of runtime you want, other accessories you want to plug into the UPS (monitor, speakers, etc.), and also whether you believe the computer is actually going to reach a 750W power draw.
A safe bet would be a SUA1500, very good UPS that could handle the full 750W power draw if needed for probably close to 10 minutes. Realistically though, unless this is a specialized computer that is maxed out internally w/ components, you'll probably never get close to 750W. I know I have a 400W power supply in my homebuilt computer, w/ 4 drives in it and I never exceed more than 150W on my UPS load. Just an example to show that while it is good to account for the MAX draw the power supply may have, keep in mind that the real power used depends on how much power your componenets are actually drawing.