What you are asking has been asked on this board before and the answer is always the same: Not really possible with your standard home router.
What you are talking about has to be done at the distribution point of your network, which is the router. I have yet to see a reasonably priced home router with this ability. People have brought up QoS, Quality of Service, but it falls short of bandwidth throttling. You can mess around with it and it might help some.
To do what you are talking about there are two options: buy a more expensive router or build your own, with an old PC, 2 network cards and some router software, normally Linux based. The more expensive router I am referring to is the Cisco 871w
it starts above $250 but eBay might have it for less. That router runs the Cisco IOS, which will do what you need.
The only other thing I could suggest is, get your own connection.