The wattage rating of a UPS for a PC depends on all the components you decide to connect for backup.. Thus, assuming you are just plugging your main rig and the monitor to the UPS, the safe wattage for a satisfactory backup session may be calculated as : 1.5 x PSU wattage (under load ) + monitor wattage ( Turned ON condition )..
Now, to answer if it is required.. UPS, AVR's and surge protectors are made for some purpose.. It not only functions to give backup when their is no power but also provides safety against over voltage, transients and surges which ( on your bad luck day ) can compromise proper working of any small part rendering total inactivity.. Does anyone want that sort of headache..!! Its a measure of safety and with safety there are no compromises.. You'd definately don't want your precious build to get hampered just coz you need to save a few bucks..!!