I had the same problem with my setup. Router downstairs with wi-fi connection to my son's bedroom. At best he gets a low or very low signal to his computer, although this didn't seem to cause many probs save for an occasional loss of connection. However, when we tried to connect his x-box by wi-fi he was continually losing connection.
After spending £60 on a wi-fi card I wasn't prepared to keep throwing even more money at this when the simple answer was a 30 metre cable routed around doors and along the skirtings. I have also since added another cable for a direct connection to his computer as well. Signal to that is now excellent!