I got my UPS last week and it's just great! Already switched to battery once because of too much electrical noise.
Anyway, when you buy one make sure you got enough watts coming out of it! <A HREF="http://www.apcc.com/resource/include/techspec_index.cfm?base_sku=BP500UC&language=EN&country_code=US" target="_new">Mine</A> is a 350Watts unit and using 173Watts of it. It gives over 20 some minutes of backup if I have monitor off, and 13-14 minutes with on.
You can buy a unit with less wattage, but that mean less time up if power goes out, less toys you can connect. Also the UPC monitor tell me that I shouldn't connect more to it (it does not mean that I can't, there are other warnings before you get the overload warning).
Another good thing to look for is brownout protection (auto voltage regulator (AVR)), it make it sure your system get the same juice all the time, so you don't get blinking monitor, or even a reboot if the power dip for a couple of seconds. Software with the UPS comes handy if you're not home when power goes out, mine shut the system down safe in case the power out too long and the UPS have only 5 minutes of juice left.
I don't know what else, I'm always going for stuff that can do more than needed (more power, more extras, more sockets to plug "toys" in). Cost more, but hoping they last longer and don't have to upgrade too often.
One more thing, never ever listen sales people about how good something would be for you. Get on-line and look it up. Sales dude might tell you that it got something that you need, but then you realize at home that it doesn't. Reading can save you pain on the arse and money. Almost got the regular 500 model and on-line I found out that the PRO USB version cost only 25$ (Canadian) more with loads of extras!!!
Good luck
<font color=red>Got a silent setup, now I can hear myself thinking.... great silence</font color=red>