To leave X you can press <CTRL>+<ALT>+<BackSpace>. This kills the X server/client (which, I can't remember). If you boot to a GUI login by defeault then the chances are that it'll take you back to your GUI login.
A better way, however, would be:
Edit (as root) the file /etc/inittab. Change the line id:5:initdefault: to id:3:initdefault:. Run the installer program, edit your X config file accordingly and then test it by typing startx. If all is OK, then re-edit the /etc/inittab file and change the 3 back to a 5. If you want, reboot - or, since this is Linux - type init 5.
Hope this helps.