I'm not sure you can restrict the BW by 8M vs. 4M, but there is something call QOS that will allow you to give different priorities to different devices/messages. I'm not that familiar with the 2Wire RG, Model 3801HGV, but a quick look as the datasheet »search.yahoo.com/r/_ylt=A0oGdVs4 ··· 3801.pdf
did not show any QOS mention.
I'm not sure if you can put your modem into bridge mode, but if you can, then you can purchase a router that does QOS that is better suited for what you are trying to do. There are many out there, but for example the D-Link DIR-657, DIR-827, DIR-857. Netgear and Cisco make some nice ones too.