What you want to do is called Quality of Service Management. This feature basically lets you determine the bandwidth assigned to each client on the network. Unfortunately, it takes a managed switch to handle this sort of service. I have seen some small office router/switch boxes that say they're managed, but they are all fairly limited, and much more expensive than basic SOHO boxes. Some Cisco routers in (or near) the SOHO class offer management, but they're VERY expensive.
Basically, you either get something inexpensive that offers few features, or you get the features, but you pay out the nose for them. It sucks, but that's the way it seems to be.