Best Signal with a Mix of Protocols...

I currently have a Linksys wireless G router in my den which is on the second floor of my house. We have three mac notebooks, (Powerbook, MacBook Pro and iBook) which are mostly used on he first floor in the living area.

I know I can get a better signal by running the antenna down the hall so it's above the living area and orient the antenna in a horizontal pattern, but the cabling etc would cost as much as a new N wireless router. The question is: if I get a new N or N1 +MIMO router will my older devices B + G be able to benefit from it's extended range? I know they are downward compatible, but will my B Powerbook just see it as a B signal in strength? Is it worth doing?

The old technology will drag the new technology down with it. One G client will slow the ( not really ) N router down to G. One B client will slow everything down to B.

You need the same technology on both ends to reap the benefits. Put a B client on that N router, you'll just have fits...

The B and G clients lack the technology to get the range benefit from the N

A B client on an N router is like towing an Amish buggy behind a Ford GT40. The limits of the old become the limits of the new.