There is no need for a Login Manager at all, if you want to keep things as light as possible. A simple startx command from Terminal gets you into LXDE / openbox following Terminal login.
However, slim is a nice lightweight GUI login manager, which can replace GDM. It's easy to install too.
sudo apt-get install slim
To get slim to work, I just edited the /etc/slim.conf file (by adding startlxde as the first in the list of sessions), and created a text file ~/..xinitrc with the contents exec startlxde
To do this:
gksudo leafpad /etc/slim.conf
Use Ctrl+H to bring the Find&Replace box up, and search for xfce4-session, which is to be replaced by startlxde. Save and close Leafpad.
Enter the following into the new file, and then Save and close Leafpad.