What I used to do:
Although your PSU says 400 watts that's the load it would pull under full power (maybe I'm wrong; correct me if I am pls) I guess that if you just had 1 or 2 HDDs in there it wouldn't be pulling 400 W.
Either way, never plug your monitor into a UPS !
The idea behind a UPS is to give the PC 5 mins or so to shut itself down gracefully, and having a monitor sharing what power is available will only considerably shorten the available power the UPS has to offer.
I always set my UPSs to force power down whatever's open or running since I'd always rather lose a few minutes work or play than deal with fragged HDDs.
Also, I once spent several hours investigating why my UPS wasn't working only to discover that I'd plugged the PC into the part of the UPS that only filters the juice and protects against spikes and dips, and wasn't on the UPS battery circuit itself at all! Had a rather red face I can tell you.....