It's very likely that 450W would suffice (at least enough time for a proper OS shut down). What processor and graphics card (if any) do you have installed? Is your monitor's wattage greater than 90W? How many VA's for the UPS? That would give me an indication for about how long your power should last.
Im still not adequately informed about VA and V and VAC and such so that is why I was asking what is a good reliable UPS system you guys would reccommend based on my needs. It is actually a 24inch tv being used as a monitor.