1.5mbps line with two users.. always limit 2nd user to 256kbps.. how?

Scenario:

Two Computers (A and B)
One Router
1.5mbps DSL Line


Goal:

Limit B to only (256kbps or approx 32kBps)
A = 1.5mbps -(minus) B's usage (w/c is !> 256kbps)

Therefore if B is not using the connection A will get all Bandwidth.

If B is using the connection B is limited to 256kbps even though A doesnt need the bandwidth.




NEED:

Router that does this out-of-the-box.

or

Firmware for WRT54G2 that will enable this.




QoS is almost there but the only time B gets limited to approx 256kbps is if A needs the bandwidth. If not, B could get 100%. Problem since B will be paying just $30 bucks.