I had 3D accelleration working for Quake III
(from Lokigames.com) on a mandrake 7.1, using 
XFree86 3.3.6, and Utah GLX, with the gart 
driver for agp on my Matrox G400.

One of the reasons for upgrading to pinstripe
was to get a nice XFree86 4.0.1, where no 
shenanigans would be necessary for 3D.  WRONG.
The X Server crashes whenever I try starting 
Quake III.

wander over to the XFree86 site...
The MGA kernel module doesn't work with
(says the DRI/DRM doc.)  2.2.x.  Argh!
Build a 2.4.0-test7 kernel from kernel.org.
according to instructions from the DRI doc.
Same thing. crashes the X server horribly.

Here are some choice bits from the /var/log/XFree86.X.log

(==) MGA(0): Write-combining range (0xe2000000,0x1000000)
(==) MGA(0): Default visual is PseudoColor
(II) MGA(0): [drm] bpp: 8 depth: 8
(II) MGA(0): [drm] Direct Rendering only supported in 16 and 32 bpp modes
(II) MGA(0): Using 25732 lines for offscreen memory.
(II) MGA(0): direct rendering
disabled                                          

The above is interesting because the screen was,
in fact, in 16bpp mode...  Here's another failure
mode:

(==) MGA(0): Write-combining range (0xe2000000,0x1000000)
(==) MGA(0): Default visual is TrueColor
(II) MGA(0): [drm] Sarea 2176+624: 2800
(EE) MGA(0): [drm] DRIScreenInit Failed
(II) MGA(0): Using 5529 lines for offscreen memory.
(**) MGA(0): DPMS enabled
(II) MGA(0): direct rendering disabled
(II) MGA(0): Using overlay video

Pouf! BSOD (in my case, Black SOD...)