NAT under Gentoo

Hi everyone.

I am trying to set up a NAT with my linux box (that uses cablemodem with 
dhcp). The idea is to setup a dhcp server for the clients machines (that 
might be running windows or Linux) to give the clients transparent Internet 
access (those clientes will only browse, check mail and use ICQ/MSN 
messenger). Some basic firewall will be needed mostly to block port 139 of 
windows shared resources.

I cant find a dhcp server and it looks like the docs at iptables.org are, 
somehow, a little advanced for me or it is more complicated to do than in 
Windows (is this obvious?).

Some pointers about the subject will be greatly appreciated.

Thanks in advance.
Jaime.