i dont use dial up. but...in theory it should be the same.
A Network card is normally called eth0 under linux. I am guessing that your modem will be ppp0 or something like that. SOmeone with linux modem experience correct me on that one.
Regardless, you are going to need a switch in which all your computers in your private network attach to.
That switch will be uplinked to your nat box. Probably connected to eth0.
So all you got to do now is forward packets from eth0 to ppp0.
You are correct about iptables. Iptables is the native program that works with the kernel to nat/route/mangle/drop/etc packets.
You can get detailed information here:
but, if you are already somewhat familiar with iptables, here are the core iptables parameters that you will need:
(substitute $EXTIF with external interface (ppp0 - or whatever) )
IPTABLES -A FORWARD -i $EXTIF -o $INTIF -m state --state ESTABLISHED,RELATED -j ACCEPT
IPTABLES -A FORWARD -i $INTIF -o $EXTIF -j ACCEPT
IPTABLES -A FORWARD -j LOG
IPTABLES -t nat -A POSTROUTING -o $EXTIF -j MASQUERADE