Unplug the router and the cable modem. Wait a minute plug in cable modem, wait until you get a signal, then plug in the router. If you have a bridge, bypass it and plug your Xbox directly into your router to determine f the problem is the bridge.
I just got a router and XBL tonight and I was having a hell of a time, but as of a half hour ago I'm in! MS tech support said there were server side issues.