If you have successfully finished the installation and configuration of the kernel and XFree, it's time to configure your display manager(s).
Beside the graphical differences, xdm/kdm and gdm handle the X servers differently. Gdm will start the X servers in the order specified in it's configuration file (and stop them in the reverse order). Xdm/kdm will start and stop all the X servers at the same time(in case there are no opened X sessions). Also, restarting the gdm daemon means end for all X sessions, but if you restart xdm/kdm when you are under X, your session won't be closed.
Using gdm could help you to retain the VGA console and prevents lock-ups with some graphic cards (check the compatibility list).
Using xdm/kdm allows you to switch it's configuration retaining your opened X session(of course the changes shouldn't affect the X server you are using).
Have in mind that for the older Preferred Bus ID XFree Server (version 1) you have to specify the desired graphic card with parameter "-prefbusid x:x:x" where x:x:x is the Bus ID of the desired graphic card. Just append "-prefbusid x:x:x" with the correct Bus ID of the card you want to start right before the last argument vt[x].