Hi Joe,
Yeah, you'll probably have a "black hole" by virtue of the antenna that high over the house.

Look at the radiation patterns of the antenna you're buying, and it may give some indication. Most of the manufactures will have them posted. For a good sampling, I like <a href="http://www.superpass.com" target="_new">Superpass</a>. They're a good company to deal with, make nice units and are reasonably priced.

A second card will cure the immediate problem. Just keep the cards on a non-overlapping channels (1, 6, and 11) and don't forget roaming. You may need to implement it on your model of AP, so you can move between the channels. To get the best receprtion for you setup you might want to have the AP mounted midway on the tower in a NEMA rated box, and have the "long range" antenna at the top for distance and the "local" antenna mounted down closer to ground level. You may still have an area where you are between the two signals, but it would probalby pretty small.

If you get this functioning, post some info on a web page, so we can all see it.

Cheers,
Thorn