3COM Lan Modem

I have a small windows 2000 server network. I have 8 windows 2000 pro clients. The server has active directory installed. I purchased a 3COM 56K Lan modem to give users access to our 1 isp account. My question is: Should I let the lan modem assign ip addresses or should I installed dhcp on the server(256mg Ram). If I let the lan modem, how do I get it to work with active directory? Please give me detail information on how to let the server know the lan modem will give out the ip addresses. Thank you.