QoS doesn't really have a "one size fits all" configuration. Every network is different, with different bandwidth available and different requirements.
In the RT-N66U, you could provide the MAC address of the Xbox 360/PS3 to prioritize that entirely, or give priority to ports 53, 88 and 3074 (TCP/UDP) (for Xbox 360) which is what I've done with my network. I don't know what ports PS3 uses, but a quick Google search will tell you.
You may find this post
that I made a little while back to be of some use. It's showing you the basics of how I configured QoS for my network.