Anyone who can give me some advices about what to look for when I buy an UPS? I have no experience with UPS.
(UPS = Uninterruptible Power Supply)

What you need to know before you start looking is the total amount of power you will be needing.
You'll find the power rating of the computer power units (usually) on a sticker on the side of a unit, you'll have to take into account the maximum power usage.

Note that you should not expect be able to power CRT/LCD displays, printers/faxes or other devices that create power spikes from a UPS, because when those are turned on their power spikes trigger the safety circuits in a UPS which makes it turn off completely (to protect connected equipment - think power surges and lightning strike protection here), defeating the purpose of an uninterrupted power supply. This is especially true for CRT monitors, Laser printers and other 'heavy' power devices.

You want your UPS to be able to signal the connected computer(s) that power has been interrupted or has become unstable, so a communication cable option is very valuable. This allows the UPS to signal the power states of itself and mains power to the computer(s), to which the computer can respond accordingly by initiating a shutdown sequence or enter hibernation to disk modes. No communication between UPS and computer simply means battery dead, computer dies with it. A 'reasonable' time in my opinion is at the very least 15 minutes, you have to realise that once power is cut, the machines will have to start with either shutting down gracefully or enter hibernation (to disk) mode, this takes time and sometimes a lot of time when services running on those machines are busy fleeing to disk with their data. If your UPS can't hold out long enough for the attached machines to complete their shutdown/hibernation, well, you know what happens.

UPS's are often labeled with things like 300VA, 800VA, 1100VA, etc. VA stands for Volt-Ampere.
Here comes a bit of math I picked up off the web:
You multiply the total wattage by three factors, giving you somewhat of an estimated power requirement of your UPS.
First factor is 1.25 to allow a bit of slack for future power needs and battery deterioration.
Second factor is 1.4 to convert Watts to VA.
Third factor is 1.4 to make sure your UPS operates at less than 70% of its rated load to prevent so called "nuisance tripping", e.g. your UPS cutting out from power spikes or your computer drawing more power to accomodate for higher processing needs for example.

So as an example you have 200 Watts to feed: 200 x (1.25 x 1.4 x 1.4) = 490VA. Since there is no UPS with an odd rating like that, the closest one from for instance APC (American Power Conversion) would be their 500 or 600VA models Back-UPS or Smart-UPS. The Smart-UPS comes with the communication option, by the way.


Hope that's of any use to you.