Info on port forwarding with IPTABLES is here: http://www.hackorama.com/network/portfwd.shtml
This is not WRT specific.
What is comes down to is:
/sbin/iptables -t nat -A PREROUTING -p tcp -i eth0 -d xxx.xxx.xxx.xxx --dport 8888 -j DNAT --to 192.168.0.2:80 /sbin/iptables -A FORWARD -p tcp -i eth0 -d 192.168.0.2 --dport 80 -j ACCEPT