You can review this, however the UI maybe different:
Gaming and QoS for XBL
Does the DSL modem have a QoS section?
If the DSL modem doesn't have any QoS options persae, then you may want to bridge the DSL modem and attached a DIR Series router to the DSL modem that has the QoS options seen in the sticky then management of gaming will be available to you to use.