did you try with "internet configuration Wizard" tool? I do not remember bec'z i m using RHEL 3, but just check with the following path.
click "start applicaion button" >> System Tools >> Internet Configuratioon Wizard" and click on XDSL connection and try configuring modem.
Once you get connected with internet, you will need to set some iptables (bec'z ur using RHL 8.0) rules to share internet.
for sharing internet and to setup rules, just search in google.com or in linuxquestions.org, you will get lots of tutorial on it.