you will want to figure out what your total draw is. the easiest way is if you have one of those outlet plug in models which gives real time power usage. if not then you will have to rely on the specs for your monitor and a guess on your pc.
once you have this information figure out how long you want to be able to sustain your pc with the power out.
total draw is normally about 280-330, max draw is in the low 400's.
i have the bbu unit from apc which is the tower type with a blue lcd screen commonly available from best buy. i have the model which is one step down from the top... sells for around $200.
it can only keep my system powered up for around 8-11 minutes. thats without connecting anything else to it such as my tv screen, audio system and other such things.
in order to fully power such a system for the same period of time i could have easily spent $500-900. it gets to be excessive on the budget for almost zero gain.