420 VA looks kinda scrawny for the machine you are building. The VA (volt amps) rating is supposed to be greater than the power draw of your system. If not, it will simply overload and shut down when the power fails. You'll get a slightly noisier crash than if you had no UPS.
If you are using an LCD instead of a CRT monitor that helps, but a 680 watt power supply demands a similar VA rating. 'Volt amps' are basically equivalent to watts, so you want at least a 680 VA UPS. A 750 is probably a good idea.
Hey, I'm using an 850 VA UPS to run a computer with an 'average' 250 watt power supply and a 19" CRT monitor (it draws a maximum of about 240 watts). Yes, it's much more than 'absolutely necessary'. That just means it lasts longer when the power goes out.
Yes, it's more expensive than the cheapest solution, but you definitely get what you pay for in backup battery power. You might want to go for a 1000 VA UPS -- 1 kw!