Re: GeForce 8600
In your bug report, I see two displays, CRT0 & CRT1. CRT0 is identifying itself as a Dell D1024TM. CRT1 isn't identifying itself at all:
(WW) NVIDIA(GPU-0): Unable to read EDID for display device CRT-1
This is likely why you're falling back to 800x600, as no valid modes are detected. However, its hard to say for certain, as your bug report was generated when only a single display device was configured in xorg.conf.
You can try setting UseEDID to false, however if you need further assistance, you'll need to reconfigure X for separate X screens, start X with the following command:
startx -- -logverbose 6
and then generate a new bug report.