Michel,
    I think they are confusing the DefaultDepth you
set in XF86Config-4 with bpp. I have...
        DefaultDepth   24
with agpgart/dri loaded from benh's current kernel
and my XFree86 log shows...
(**) R128(0): Using AGP 2x mode
(--) Depth 24 pixmap format is 32 bpp
                     Jack