[SLUG] X won't play with me anymore

[David Fisher <david@xxxxxxxxxxxxxxxxxxxxx>]

Sorry 'bout the previous empty message, I'm not used to mailx.
Due to a recent death in the family (ie my rusty trusty Matrox Mill G200)
I have had to install a borrowed S3 ViRGE DX.  I have rerun xf68config and now 
when I try startx as user david I get 
X: user not authorised to run the X server, aborting.

Now I know there's a fix for this but I cannot for the life of me remember
it.

Anyone help, please?  We have Debian Woody, Xfree86 4.0.2.

TIA,
David (lost without EXMH)