One word of caution about a UPS - whether it's worth getting one or not really depends on how reliable your power is. Over here in the UK where power cuts and surges are really very rare these days, it's just not worth it for the majority of applications. The chances of a cheap UPS failing completely (and therefore shutting your PC down whether mains electricity is available or not) is in most cases far higher than the chances of a power cut / surge.
Of course, if you're plugging it into somewhere where the breakers are always tripping (though I'd question WHY they were always tripping, this really shouldn't be happening!) then that's a different story - it's all about how reliable the power is that your computer is using.
In the real world when we've got something that's pretty critical, we'll feed it two power supplies, make sure it can change over from either seamlessly, feed one of the power supplies directly into the mains and the other into the UPS. That way if your UPS fails, you've still got power - the only way you'll lose it is if both the UPS and the mains fail at the same time (which has never happened to me yet!)
