You need a router w/ QoS (Quality of Service) controls. Features vary widely among routers (some don’t have it at all). I personally like tomato third-party firmware, which can only be installed on a finite number of routers. It's a surprisingly complex subject (sounds simple, just split the traffic, right), but there's a lot of things to consider. The following document might help you understand the issue and make a better decision. No point in me repeating it all here.