Where is the base station in relation to the floor? If you place it higher up, it should do better, because the signal won't have to go through so much furniture. Also, since it's the dual ethernet model, you can get an extension antenna that works a lot better; the basic antenna in that model is exactly the same as in a PCMCIA wireless card, and thus not as good as it could be. You could also try rotating the base station a bit (maybe 1/8th of a turn at a time) to see if its antenna orientation can be improved for your situation.
In addition. look at the direction the iMac is facing. I believe that the antenna is in the user's side of the base of the iMac, which means that if the computer is facing away from the AirPort Base Station, then the bulk of the computer is between the antenna and the base station. Try rotating the iMac a bit and see if reception improves.