"Extend Wireless Network" Question.

The client devices will connect to the base station that gives them the strongest connection; from the user point of view only one network exists. The decision about which base station to connect to is done behind the scenes.

If the second base station is close to the PS3, it's highly unlikely that the PS3 would make a connection with the one downstairs since it's logically always going to get a stronger signal from the one upstairs closer to it. If it does though, you won't know about it without querying your Airport devices, and it's in the best interest of the PS3 because it means the lower base station is giving the best signal. But, as I said, that's unlikely.

Simple answer: the "Extend Wireless Network" protocol is designed to 'just work', don't try to force anything. You can't, and you really don't need to.