I am setting up home network with four PCs. One is in the office and
is connected to an 802.11n wireless router via a network cable. Two
others have 802.11n wireless cards. Unfortunately one PC is in
opposite side of the house floor above from the router and the signal
is very weak. I am not sure it is due to the card or it is just
because there so many walls between the card and the router the signal
is very weak and transfer rate is very low.

What's the possible solution to this problem? I am thinking about
running a network cable from the router to the room where the PC is.