The upstairs pc will talk to the router, not the other computer for internet access.
So you could hae 5 different operating systems running, as long as they can talk to the router they will be fine.
As for the PS3 talking to the Ubuntu PC, yes it can be done. I dont have the names of the programs at hand, but a quick google should come up with those.
I remember when I was looking around there was about 3 or 4 highly reccomended ones to use for this application.