Recently I did a little experiment on running G450 on an XP box. Surprisingly the latest driver does not have any 3D support, so I decided to launch a test over different versions of G450 drivers and check their capability on D3D and OpenGL support. It turns out Matrox pulled off 3D support for G450 sometime between versions 5.93 and 5.95.

Here is the description of the test. Platform:
AMD K6-3+ 450MHz OC 100x6, Pentium MMX 233MHz OC 100x3
ASUS P5A-B
128MB x3 ECC PC100 SDRAM
Matrox G450 AGP 16MB, PCI 32MB
Two Dell ultrasharp 1907FP
Test tools: dxdiag for D3D, Quake3Arena for OpenGL

I did all the tests on both AMD and Intel CPUs to rule out the possibility of processor incompatibility, and they turned out to be identical. The AGP and PCI versions have almost identical outcomes from version 5.82 and above. Clean install/uninstall are performed for each test case. The table shows the results under dual monitor mode.

How to read the entries in the table:
Version number: versions are taken from the .inf file. Matrox has a separate line of version numbers for driver packages, but different packages may actually carry the same driver for G450. So I think the version number from .inf file should be a more accurate description of the driver.
N/A: not supported.
Pass: passed dxdiag D3D test.
D3D/1.1.3: the OpenGL driver used in Q3A. D3D means it actually uses Microsoft D3D wrapper, and 1.1.3 means it uses Matrox OpenGL driver and the version is 1.1.3.
Blink: screen blinks and nothing further can be inspected.
VGA mode: I can't get the XP stock driver work on PCI version of G450 except the primitive VGA mode.
L/R: the position of monitors in dual monitor setting.

I have also tested other cases that are not included in the result table. These cases are identical to some cases in the table and I will describe them below.
Enable only one monitor under dual head mode: each port will act as if it is positioned on the right in dual monitor settings.
Disable dual head mode: only port 1 has output and it acts the same as being positioned on the right.
Panoramic mode: only port1 has output under 3D and it acts the same as being positioned on the right.
Mirror mode: can't find a way to get there...