I'm not sure if this is the easiest way, but it will definately work. Edit the /etc/inittab so that this part looks like this:
# Default runlevel. The runlevels used by RHS are:
# 0 - halt (Do NOT set initdefault to this)
# 1 - Single user mode
# 2 - Multiuser, without NFS (The same as 3, if you do not have networking)
# 3 - Full multiuser mode
# 4 - unused
# 5 - X11
# 6 - reboot (Do NOT set initdefault to this)
Specifically, all you need to do is change the middle number in that last line from X11 to Multiuser. Then reboot and it will start in the command prompt.
From there, to get into X you can type 'startx' and if you want it to always log you into GUI, just edit the /etc/inittab file so that the initdefault is X11.