thanks for the help, though it's not working, i must have done something wrong. A couple of questions:
do i use eth1 instead of ppp0 (my cable modem is connected to eth1, eth0 connects to the network, and in the line
-A forward -i ppp0 -s <ipaddress>/<netmask> -j MASQ
i use the ipaddress and netmask of eth1? if that's right, then i'm pretty sure i did everything right, but i'll double check. Thanks again for the help.