There are a couple of different steps needed to setup up QOS on this router so that your gaming packets take priority.
First: Make a DHCP reservation for your 360. To do this go to your System Settings on your XBOX dashboard and then then click on Network Settings. There you see it, something like 192.168.0.103.
Next Login to your DIR-655 web interface. Navigate to the SETUP Tab, Then click Network Settings on the left. Scroll down to Add DHCP Reservation. Name it XBOX, enter the IP Address (that you looked up on your XBOX) and copy the MAC from the table below that section (Dynamically Assign Clients). Copy the MAC to the left of the IP of your XBOX, and paste it into the MAC Address Box and hit Save.
Second: Go to the ADVANCED Tab, Then click QOS Engine on the left. Enable everything, the scroll down to 10 - QOS Engine Rules. Then enter the information as seen below:
Hit Save on top of the page and you are done!