My previous post was to let the system boot into FC6 and when the boot is completed press <CTRL><ALT><F1> (or F2 etc). If that did not work, you can tell grub to start FC6 differently.
When the grub screen appears, follow these instructions
No GRUB around here at this momnet, so I could not try it.
If RH/Fedora did not change too much since the last time I used it, you can pass 3
as the runlevel so FC6 boots in run level 3 (fully functional text mode).
Next you can login as root and make this change permanent (if you want to) by editing /etc/inittab and change the initdefault.
And now you should be ready to change your X configuration.