This is not meant to be a guide on how to set up your specific monitor, or videocard. In fact, I assume that you already have X windows running for your setup. Please refer to the XF86 Documentation for more information.
As far as I know, there are no limits to which video cards you can configure this way, nor does it seem to matter if you mix different types of video cards in a setup, The sample configuration I use in this documentation uses two different video cards, a AGP Fire GL 1000 and a PCI Matrox Millenium II. What effect this has on 3d Accelleration I don't know as I don't currently accellerate either of my video cards.
This Howto assumes that you know how to edit text files, do basic video card configuration for X Windows, add and remove hardware from your system, start and stop system services, and follow simple instructions. If feel you will have trouble with any of these things, please seek help. I am not responsible if you damage any of your stuff.