Try with updating /etc/driver_aliases with the PCI-ID for the card and then run sys-unconfig.
The BGE driver is documented to work with 5751 in the man page for bge(7D) so I would try to add this Motherboards PCI-ID to the list of BGE chipsets in /etc/driver_aliases. You find the PCI ID with either /usr/X11/bin/scanpci or /usr/bin/prtconf -v This message posted from opensolaris.org
