control panel -> screen resoultion -> right click monitor you want adjsuted -> properties.
I got HD4870 and 2 monitors - one is my main display 22' samsung and my 32' bravia is connected as a secondary.
each has a differnet resoulotion and I constantly switch betweem them and extended mode.
7 works very very very well in this setup. perfect actually.
winkey+p switch between monitors (aweseme for me)
shift+winkey+left/right arrow - move open app to other monitor in extended mode (yes!).
using 8.612.0.0 driver
from windows update (the one from ati worked fine before too)
hope this helps.
i love 7.