You may want to look at something along the lines of a line-conditioning UPS.
Some of the straight battery backup UPSs tend to not switch fast enough to the battery to prevent shut-down and data loss. We've had a member that had some severe trouble with this problem due to the sheer number of power interruptions they encountered.
Plus the Macs tend to be somewhat sensitive to line noise and brownouts (low line voltage due to overloaded circuit, or power company.)
And the big PowerMac and a CRT are probably going to require the bigger UPSs than people typically buy or have in the closet.
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There are a few older threads on this topic, so a search should yield some data -- I've posted in quite a few of them so an advanced search on UPS and Member/Sun Baked should narrow it down to some of the juicy threads that included some of the actual Watts used real quick.
Probably the 1000-1500VA UPS for a start, depending on what else you'll be plugging in to it -- but, it's the maximum output wattage that's key, not the VA.