There are a few things you can do, like altering that direction the PowerBook faces and elevating the wireless router so there are fewer obstructions between you and it (mine is on a fairly high shelf and I get much better coverage because of it).
But rjt1000 is right; the metal case on PowerBooks interacts with the wireless antennas and messes up their performance. The aluminum cases are less of a problem than the titanium ones, but it's still a problem-and there's probably a similar problem with MacBook Pros, too.