For 1000/Full you do not need to configure the interface from default.
For 100/Full use ndd to set the speeds, use something like this script at /etc/rc2.d/S05bge_settings (don't forget to chmod +x)
#!/bin/sh # Set kernel parameters pertaining to bge devices NDD=/usr/sbin/ndd case "$1" in "start") $NDD -set /dev/bge2 adv_1000fdx_cap 0 $NDD -set /dev/bge2 adv_1000hdx_cap 0 $NDD -set /dev/bge2 adv_100fdx_cap 1 $NDD -set /dev/bge2 adv_100hdx_cap 0 $NDD -set /dev/bge2 adv_10fdx_cap 0 $NDD -set /dev/bge2 adv_10hdx_cap 0 $NDD -set /dev/bge2 adv_autoneg_cap 0 ;; *) echo "usage: `basename $0` start" ;; esac