My UPM power meter measured at the most just over 400W when i had my HTPC (while crunching Rosetta) + PS3 + DVD Player + Receiver + Digital Cable box + my precious 46" Sharp Aquos. Everything was on. I probably didn't have the PS3 actually playing a game or a DVD spinning or music blaring, but that's a rough guess for you.
I have an APC 1300VA
NCIX.com - Buy APC BACK-UPS RS 1300VA 780W UPS 8 Outlet 340J LCD Display RJ-45 Protection - BR1300LCD In Canada. and it's just powering my network gear + fileserver + PC router. I don't know the load on it right now, but I know I can't hear it.
PS: Holy jeeze the markup on those UPS's is insane. I bought this from Futureshop on staff pricing while it was on sale and it wasn't much over $125 if I can remember correctly.