I had same problem with a HD install with a UXGA screen. The solution was to do a CD session, and choose your resolution at startup, let the cd session boot, and then if this session runs with the desired resolution, copy the XF86Config and XF86Config-4 files from the ram disk of your successful CD-install to somewhere safe. Then reboot using your HD install and replace the two HD-install files mentioned with the CD-ones. On reboot the right resolution comes up. Worked for me. (I am sure this is not the quickest way!)