First, look at /var/log/Xorg.0.log and confirm that X is detecting the chip and using the mga driver.
Matrox seem to have discontinued the Forum at their website, and the only "legacy" drivers they offer for the 450 are older versions of the mga driver that Xorg ships.
Read the manual page for mga. The only option that might help is "AccelMethod" (Try "EXA" instead of the default "XAA"). I disclaim any responsibility for the outcome of this, I haven't tried it.
I am using Arch Linux at the moment, and here glxgears reports 335fps - twice as fast as fedora. This might be due to the new Xorg 1.8 Arch just updated to. Maybe you should reinstall Debian?
If you want 3D performance, buy an nvidia or ati card.