|Thanks for the replies..|
So I changed the setup for 2nd floor PCs to this: (since it will take a while to find a crossover cable around here..)
DSL Modem <> Wifi Router <> 4 port Switch <> PC X&Y (via LAN)
4 port Hub = Linksys SD205, which has MDI/MDI-X detection
Now both PC X and PC Y can't get DHCP ip address (169.254.x.x)
What seems to be the problem?
edit: Solved, it's bad cable between Wifi Router and 4 port Switch, should've checked this earlier..