Hi! Before saying anything else, I feel like mentioning I've searched for simmilar threads, but didn't find anything.
So, the sittation goes like this: I've bought a second monitor I want to use for chats/browsing/etc while using the other one for gaming, or something else. The new (and currently main monitor) is 1920x1080 , and the second one is 1680x1050. I've installed it, but after I did my stuff in the screen resolution thing , I've noticed the main monitor isn't showing the full image. It has a black border, I think it is set to 1680x1050, and that thing is to correct the missing space.
I'm guessing your new monitor is connected via HDMI correct? If so, open Catalyst Control Center -> My Digital Flat-Panels -> Scaling Options -> and increase Overscan.
At least that's what fixed it on my Samsung TV that I'm using as a monitor.