I'm pretty sure it's due to your Video setttings in the Catalyst Control Center (the ATI Video card drivers). First of all (and this is very important), make sure your drivers are up-to-date. Download the newest catalyst drivers from ATI (though if you use G-Force you might want to only update to version 6.5 rather than to the current 6.10 because the newer drivers break some stuff in G-Force for now). Anyway, the newer drivers fixed some stuff related to this so make sure you have them installed.
Open up Catalyst (right click on the Desktop somewhere and choose Catalyst Control Center).
Then, when it's open, make sure you go to Advanced mode (click on the "View" button at the top left and choose Advanced View, or pick Advanced if it gives you the option). Then, in the tree on the left choose Video --> Theater Mode. On the right in the Graphics Adapter drop down menu (at the top) choose whichever graphics card has the second monitor attached to it.
Then under "Clone Mode" choose Same on all displays, and choose in Standard Mode under "Extended Desktop".
Then choose your other graphics adapter from the drop down and make sure the settings match for it.
Then make sure MC's Display Settings are set to:
Full Screen, Monitor: Nearest Monitor
Full Screen, Resolution: Desktop Resolution
You should be all set then, I suspect.