What you really want is a real router, one with some form of priority queueing. Basically what this will let you do is allow bittorrent traffic through, and allow it to take up the whole upload/download speed, but prioritize it lower than other traffic (you could limit it like this based on tcp/udp port #). This way, when bittorrent is going full tilt, and the router sees a packet that's related to say, web browsing, it puts the bittorrent stream on hold for the few milliseconds that it would take to download the web page. The net result is that bittorrent takes up 100% of your bandwidth except when you are doing something else.
There are a few routers that are'nt that expensive that can do this. My preference is making your own router out of an old PC (a 100mhz machine and 2 100mbit network cards will do fine, anything more than that is just gravy), using the OpenBSD OS, and correctly configuring pf to route packets in this manner.