Hi FrodBonzi,
With the crazy weather these days around the world, you never now when the power is going to be interrupted. So I feal a good UPS is a must. I've been using this one for the past 4 years with no problems.
http://www.newegg.com/Product/Product.aspx?Item=N82E16842101419 I've replaced the internal gel-cell batteries twice during this time to maintain top reliability from the UPS.
Here's APC's website with a calculator to help in choosing a UPS for your needs.
https://www.apc.com/products/family/index.cfm?id=27&tab=modelsA Watt Meter like these
http://www.newegg.com/Product/ProductList.aspx?Description=kill-a-watt&Submit=ENE will help in knowing the wattage draw of your system.
In using the watt meter, plug it into the wall and connect your PC, Monitor, Modem, Router, etc. into a power strip and then connect the power strip to the Watt Meter. The Watt Meter will tell you what the combined devices are drawing at the plug that you need covered by the UPS. The UPS I suggested should be more than enough for your needs.
I hope this helps you in making a decision.