If you attach the mediacenter to the 655 router, and then turn on DMZ under
Advanced->Firewall And set it to the mediacenter computer, you should be able to share the mediacenter to all the computer within your network.
The guys around here probably know a better way, so you might want to hang tight for their response before you go moving your mediacenter

Also, I can't actually see why you are wanting the 655 to act as a router.