Bandwidth throttling is generally best managed on the router or perhaps a layer 3 managed switch. Some less expensive routers, like Cisco/Linksys will support firmware hacks like DD-WRT that may allow you to do something like this, but I would first do some research to make sure that such a feature was include in the product. Some routers will support a similar type of application throttling with their QoS (Quality of Service) applet, but I haven't tried or tested it and your router may or may not contain that feature.
There are some software solutions out there called Traffic Shapers, but they are generally required to be installed on each machine on a peer to peer network or on a network server acting as a gateway/proxy device between the clients and the internet. I don't think the Xbox would support any such type of installation.
Try googling bandwidth throttling or traffic shaper there is quite a bit of information out there, you may find something that fits your needs.