General :: Bandwidth Management - How To Limit Connection Speed

In my household a number of people use the internet. Up to a maximum of 3 wired connections and 2 wireless connections at its peak, all connection through my D-LINK G604T router. The problem is, when one person is downloading or watching ..... or whatever, the others using the internet suffer. I've spent hours configuring QoS on my router, and long story short, no matter how I configure it, it just simply does not work. QoS in no way shape or form limits connection speed (which it says it should). Anyway.

I have a spare computer under my desk, and I'd like to know if I could set this up with a (free) linux distro that limits bandwidth speed per connection. For example, of the 1500 kb/ps (about) my modem pulls, is there a way to limit that to 768 or 512 per connection? so person A can still download, person B can still watch ....., and person C can still play counterstrike with a latency under 100. This would solve many, many arguments in my house I am *fairly* good with computers, but if the distro came with documentation and a GUI that would be awesome.

* TL;DR: *
Is there a linux distro I can load on a spare computer that limits bandwidth per connection, wireless or otherwise, with good documentation?
Failing that is there firmware I can use for my modem (dlink g604t) that would do the same?
Failing that do you know of any good hitmen that would solve my family arguments, ahem, permanently?