first i'm not sure there ever was a 968 (you put 986) diamond stealth ?
i got an old trio 64 going once using XFree3 instead of XFree4 once
XFree4 was a major change in the structure of x and i think some of the older support never made the transition but i'm not sure about that
the changes in xfree at that time were totally coppied into xorg as well
(it was before xfree was evil) *joke*
here is a list of supported chipsets for xfree 3.3.6