I'll admit, I don't understand, so if anyone wants to clear this up for me, props to them.
I went through the dselect command, installed 96M of stuff (didn't change anything though) of which xlibmesa3 and its affiliates were included. No dependancy problems there, modutils not removed.
So why, in apt-get, does modutils (amongst a couple of others) get removed when you apt-get install xlibmesa3, but when you dselect, no such thing occurs?
By the way, nvidia-glx and nvidia-glx-dev were then installed, and everything works now.