Not sure I would have described the G400 (Matrox G400 right?) as open-source friendly. IIRC Matrox used to ship a black box binary that was needed to make various features work, without it you got a 2D framebuffer and some quality DACs, which aren't very relevant at this point because everybody is upgrading to digital video...
On my Fedora 16 box with 3.3.4, I seem to have a kernel module named 'mga' which claims to be for "Matrox G200/G400" and requires binary firmware I don't have (I probably have a G400 somewhere but no slot to put it in) but I
- don't know for sure that this works with your specific model (of course)
- don't know if it enables sufficient features to make a modern glitzy UI work properly, e.g. GNOME 3