After activating QOS on my WAG54G2 DSL router everything seemed to be fine, but after restarting it a few weeks later the overall internet speed of all our pc's went down dramatically.
I checked for a firmware update, but there was nothing on the webpage of Linksys. So I decided to restore the old settings (without QOS) and it worked immediately - even without restart of the WAG54G2 router. I even deactivated "WMM-Unterstützung" and it still works perfectly:
Hope it will work for you too ... ;-))