I don't have rhel 5. In Fedora 11, you would run "system-config-network" and click on the "Add" button and select xDSL. You may be prompted to install the rp-pppoe package if it isn't installed already.
I have read that your modem has two modes, DHCP and bridged. Are you certain that it is in bridged mode? If it contains an internal router, and dhcp server, you can simply use dhcp to have the IP, netmask and other items set. The pppoe connection is between the modem and the ISP in this case. The connection to your computer is a normal ethernet connection. You may need to connect to the modem with a browser to authorize the modem. I think this is what Visesh was referring to.
If you know this isn't the case, then make sure you have the `pppoe' kernel module loaded and run "system-config-network".
This link may help as well:
http://kbase.redhat.com/faq/docs/DOC-8566