IIRC, runlevel 6 causes the system to reboot (try typing "init 6"; it will work the same as "reboot" or as pressing CTRL+ALT+DEL). If, when your system boots, it comes to a graphical login, you want to edit level 5. Otherwise, edit level 3. To make the job easier, try using "linuxconf" to "control service activity". It is really much easier.
HTH,
Josh