boost signal with second router?

I am a realtor and I just moved upstairs from my office. I have a wireless router (a) in the basement, three floors down, and another one (b) up here. Is there a way I can pick up the signal from (a) and direct it into (b) which I would use with my Mac tower. My laptop has Airport, the tower doesn't. I don't want to run a cable up here, and my broker is well aware of what I am trying to do.

How can that be accomplished? Router (b) is a Linksys WRT54G. Not sure what router (a) is. We have a highspeed cable connection, not dsl.

Any assistance well appreciated.

Cheers.