UPS (Uninterrupted Power Supply) and AVR (Automatic Voltage Regulator) are both overkill for gaming rigs. UPS & AVR are requirements for computers where data preservation is paramount, and for areas that have frequent power outages.
As far as the PSU is concerned, I would recommend a good quality 80+ silver certified PSU of 750Watts or higher for gaming. I have a Corsair HX-850 modular PSU. The video cards used in gaming rigs tend to draw quite a lot of power depending on the type of card. My GTX-275 video card draws about 219 Watts without SLI. If you use SLI/Crossfire the power requirement will be even higher.
If you still feel that you want a UPS system, then the rating of the UPS system must be at least equal to the total wattage of the gaming rig; certainly more than 300 Watts.
Hope this helps.
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..!!