the best way for you to go and read about System Init:
And about network configuration:
modprobe eth1 will not work.
first of all during init there are very few environment variables are set so you probably need to have explicit path:
/sbin/modprobe <module_for_your_card> :P
but this is actually written for you in the /etc/rc.d/rc.modules
you just have to find out what module you need for your card and uncomment apropriate line in /etc/rc.d/rc.modules.
initialization of you interface then should be done ni /etc/rc.d/rc.inet1
something along the line:
ifconfig eth1 YOUR_IP_ADDRESS up (plus some other optioins you might need)