I already got a lot of the necessary information from a post by Pat Regan that you should definitely read before starting to play around with your settings!

At first I tried to configure my new QX2710 with my old monitor connected, but I figured I might aswell just do it remotely. So I connected to my computer from my laptop via SSH and played around with the xorg.conf. After applying the changes I’d restart lightdm (sudo service lightdm restart) until it worked.

In the meantime my monitor showed me this:

I ended up using HorizSync 89.52 - 160.0, VertRefresh 59.96 - 120.0 and a Modeline generated by cvt. Here’s the xorg.conf.

Here’s a comparison of screen resolutions before - after: Desktop background showing a 1680x1050 rectangle

I can finally watch 1080p videos without downscaling!1!!