I got the dual head stuff working in the following way:
- downloaded the GeForce 7600 GT drivers for 32-bit Linux from the NVidia website.
- drop into runlevel 3 as root and run the install program from there (after giving it the chmod +x).
- CentOS' native display config program chokes and screws up the xorg.conf. Ignore this and let it pick some cheesy default setting.
- Once logged in in cheesy settings, run 'nvidia-xonfig' to get nvidia driver running. Use the GUI to activate monitor 2. Logout and log back in.
- Run 'nvidia-settings' to set up the dual head appropriately and hit the radio button for Xinerama. Log out and log back in.