Problems with framebuffer and Matrox G400 dual-head

Hi all,
I've just installed on my computer a "new" Matrtox G400 dual-head and I 
would like to run framebuffer on it.
in my GRUB configuration I passed like a kernel argument 'vga=792'(this 
works with my Riva TNT2 and with a Matrox G200)and at boot i saw this:
_____
You passed an undefined mode number.
press <RETURN> to see video modes avaliable, <SPACE> to continue or wait 
30 secs.
Video adapter: VESA VGA

MODE:        COLS X ROWS
0   0F00         80x25
1   0F01         80x50
2   0F02         80x43
3   0F03         80x28
4   0F05         80x30
5   0F06         80x34
6   0F07         80x60
7   0F0A        132x43

Enter mode number or 'scan':
_____

I have this problem on linux 2.4.19 and 2.5.51
I tried to use matroxfb(on 2.4.19), i tried to use vga=318,  like 
explained in vesafb.txt, but i have always the same output.
How can I enable fb with my Matrox G400 d-h?

Se you

    Moreno




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