The Matrox G400 is just that card NOT the G450 which i have not tested.
The card can be had at the moment from H.P. online parts stores P/N 
203627-001
This was private labled for Compaq by Matrox. You said:

>I tried an Inno3D Geforce 2 MX400 card in my 164LX:

>  vga0 at pci0 dev 6 function 0: Nvidia Corporation GeForce2 MX (rev. 0xb2)

>It doesn't work as a console (no video output) and the machine crashed

Did the card initialize in SRM console or did you boot in serial mode?
- the 'no video output' is ?
Have you looked at /var/log/xdm.log to see if your error is other than:
xdm error (pid 375): Cannot open "/usr/X11R6/lib/X11/xdm/libXdmGreet.so"
while loading /usr/X11R6/lib/X11/xdm/libXdmGreet.so ?
see: http://mail-index.netbsd.org/tech-x11/2003/11/12/0000.html
alpha_video.c hack is just part of getting XFree86 to 'just work'
on alpha., ../os-support/bus/netbsdPci.c and pci.h need work.
When i or any one else gets the 21264 chip boxes to run XFree86 then its 
time to commit patches. Ogle runs for me at 11-16fps on this box, with 
option SHMMAXPGS=16384 in my kernel, which you probably have as well.
(specific DVD is a part of it...no way to generalize)
This is with a Nvidia 5200 or Matrox G400(defined above) The CPU is pegged,
there is a limit to the 600MHZ 21164 chip on clock/buffer/pci_speed. mplayer 
says 'cpu is too slow to play this..' well still crappy..1/2 speed but better
than zero speed. BTW G400 exits->xinit->exit cleanly, mouse warnings present
in /var/log/XFree86.0.log
-Mel