How to set modeline under xorg server 1.8.0

[Another Sillyname]

Hi

I have a setup which I've just upgraded to Fedora 13 with Xorg server
1.8.0.  The setup has a Quadro FX 3400 card with an IBM T221 3840 x
2400 monitor.

Under previous iterations of xorg i've output to the monitor as a
dualhead setup using 1920x2400 + 1920x2400 and it's worked pretty
well.

However under the new version of Fedora the xorg.conf file now gets
re-generated every reboot using udev to detect devices if I've
understood the docs properly.

Unfortunately the display is getting detected as a 3840x2400 screen
and is being output to at the wrong refresh rate, 50 hz rather then 25
hz, at least according to xrandr -q, and as a result the screen is
blank.  The xorg session is running however as I can vnc into it and
it's running fine.  I am using the nvidia driver from rpmfusion.

Given that xorg.conf is re-written every reboot how can I add both a
modeline and dualhead type config so that it 'sticks' after a reboot?

Thanks in advance.