For home use, the maximum wattage that could be used by a device should
less than the maximum wattage output of any UPS.
If the UPS is rated in VA, then multiply it by a nice round 0.7 for the
absolute maximum wattage it can handle.
Then you really want some extra room for error and longer ups on
battery run time, so always allow some spare capacity.
Since most 400watt P4 power supplies do not use quite that much
current, as James Sweet noted, having a single computer on a 400-500VA
UPS is about right in most cases. If there is any question about the
power used by the computer, then size the UPS based on the maximum
wattage the computer could be pulling, 400/0.7 = 571, so a 600VA or
larger should be used on just the computer.