I was having the same problems with my Dell c840 as reported by a lot of people in this forum (display corruption due to an incorrect EDID). After banging my head against the wall for two nights straight, I did some reading and created a new EDID file.
I took the existing EDID from nvidia-settings and loaded it up in the phoenix EDID editor. Phoenix is a simple freeware Windows app, but it worked just fine in wine. It's not the most intuitive app around, but it got the job done. I used the nv-driver Modeline as the basis for my changes, exported as raw, and now x.org works perfectly.
My Fedora Core 6 xorg.conf contains only two extra directives:
Option "AddARGBGLXVisuals" "true"
Option "UseDisplayDevice" "DFP-0"
Option "CustomEDID" "DFP-0:/etc/X11/nvnew.raw"
X.org 7.1 is sweeeeet. Anyway, I hope the attached EDID file can be of use.