The AGP specifications define the following...

      agp2 agp3
000   BAD  BAD
001   x1   x4
010   x2   x8
011   BAD  x8
100   x4   BAD
101   BAD  BAD
110   BAD  BAD
111   BAD  BAD
Signed-off-by: Dave Jones <davej@redhat.com>