Are you editing the XFConfig-4 file, or the XF86Config-4 file? If it doesn't say the latter, I think we've found your problem. The other possibility is that maybe XFree is using the XF86Config file (no -4) and that's why it won't use the resolutions you specifiy? Check it out and lemme know?
Celeron 633, Running at 790Mhz w/83Mhz FSB.
Currently running Slackware 8.0
etc, etc, etc.