If I havn't forgot something, Glx-module in nvidias is GLcore in nv, and is provided by mesa.
But anyway, if you want more than 5 frames per second, you need nvidia.
This is what you need to set up when you configure xserver-xfree86
(The window where you select XFree86 server modules)
dpkg-reconfigure -plow xserver-xfree86
If you have a set up that works with nv, don't touch anything else the window where you chose which video card driver you want to use AND the window where you choose which OpenGL-drivers (in server module window).
nv => GLcore and dri modules (and NOT glx)
nvidia => glx (and NOT GLcore and dri)
This should give you a working OpenGL set up. One hardware optimised from nVidia (nvidia) and one software driver from XFree86 (nv).