I'll add a few small tidbits too. - Signals on different frequencies do not intermingle. Actually on the same frequency they don't intermingle either in mid air. Only when a receiving antenna gets different signals in the same frequency space at the same time they usually interfere each other. - A site survey software like Ekahau can easily predict the distance if you use a square or circular map showing distances. I made one a while back but it's on my work laptop. Maybe I'll post a screenshot of that later. - It is more important to have your antennas perpendicular to the ground. If the antennas are upside down or not usually won't make a big difference in dipole antennas. AP's with internal antennas will have a big difference. - Using a focussed antenna is a better way to improve the distance of your signal because it helps in send AND receive. So your signal travels farther in a single direction on a directional antenna but also the antenna is more sensitive to upstream signals from clients. However if you try to improve your distance by cranking up the power you'll probably run into situations where your client upstream signal will have trouble reaching the access point since the client is usually lower powered than the access point.
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