I dunno who gave you the modem, but I agree you should do this:
modem>router>Mac.
With ATT and some other providers I've used, it's best to then setup the modem, as far as possible, to do nothing but pass traffic on to the router. No firewalls, port forwarding, routing (except to the router) or wifi. Let the Extreme do all that. I have my 2Wire set to do just the minimum, and then use 10.0.x.x addresses from the Extreme. I can then avoid double NAT problems and everything is happier with unicorns and rainbows.
Rob