qos gateway

hi
i want to setup a mikrotik box in and out bridge want to place it between my pppoe servers and the main router

i want to use it to give priority for certain ports as http and msn before other things , alli have is to mark all traffic coming from in interface and nat it to the out interface and give the priority in queue is that enought or i should do anyting else ?

pls if anyone did it before i need some help thanx