If I understand correctly you want to use your slack box w/2 nics to act as your router your your internal lan, correct? Connect your modem to eth0 on your slackbox. Eth1 should go to you router/switch/hub. All of these should be regular patch cables, not crossover. From eth1 you need to go into the uplink port on your router (not the port labeled internet). Now all of your clients on the lan point to the slackbox as their gateway, and the slackbox points to you ISP. Here is a little diagram.
modem>>eth0 (one patch cable)
eth1>>uplink port (one patch cable)
router/switch/hub>>clients (patch cables)
if you want to use dhcp you assign address to your client machines use dhcpd. this is the DHCP server that comes with slack, use either dhcpcd or dhcpclient on eth0 to accecpt from modem.