The file you want is /etc/X11/xorg.conf ... it is pretty self explanatory.
The ATI card will probably not work.
The "unsupported" (by the open source community) nvidia driver is providing you with 3D acceleration and hardware DRI. It won't affect the available resolutions which are autodetected when X is set up. Note: it is possible
that the video card will not support your screen resolution at the full color-depth.
It will also allow you to enable the desktop effects (compiz) and run beryl.