Huw -
The reason for only 5 check boxes has to do with the way .11n works. One way for increasing throughput is to make use of reflected signals. When a radio puts out a signal, the waves bounce off of hard objects and generally end up creating another copy of its signal. This copy is out of phase, travels a different path to the receiver, and is out of time synchronization. For a non-802.11n client or AP, this copy shows up as noise or a reduced signal depending on what phase it is in. Therefore, with two Tx antennas and three Rx antennas, the AP can now make use of this characteristic of radio physics by using one of the two Tx antennas to intentionally generate another copy of the signal and send it out milliseconds behind the original. However, it can use this second signal to encode DIFFERENT data. Using complex hardware and software, the AP can put this signal back together properly and decode the data. The three Rx antennas help more than just two because the AP is stationary, and it's not known from which direction and at what angle the copy of the signal will come in at. Although there's a total of six antennas (three for 2.4 GHz and three for 5 GHz), as the controller GUI indicates only 2 are used for Tx and three for Rx at any one time.
To comment on your signal strength vs. coverage question, I'm not sure what antennas you're using, but if they are omni directional, you might benefit from some directional antennas. They have a smaller footprint, but are often higher gain. If the area of use is a rather sectional area, this might work out for you.
As for the mobile computers being only 802.11g, then you would not benefit from adding any 5 GHz antennas. You might want to consider turning the 5 GHz radios off, unless you plan to offer some kind of connectivity to clients that are 5 GHz capable. Furthermore, with only one radio active, the 1250 AP can then be powered by 48v PoE (15.4 W) as opposed to the 56v PoE (30+ W) supplied by the special power injector for 1250s.
Regards,
Scott
