Both radios operate independently within their own bands.
Both radios operate at a 54Mbps *signaling* rate, which yields a theoretical total throughput of approximately 22-27 Mbps (variance due to the frame size - header/payload ratios, etc.)
That theoretical throughout will be shared to all wireless clients associated to the Access Point. Even if a client does not have traffic, it will consume some small part of the available bandwidth in administrative and maintenance traffic.
In an 802.11g system, the introduction of an 802.11b client will further reduce the total throughput, whether it "talks" or not, because the AP must communicate in a mode that prevents the "b" clients and "g" clients from stepping on each other (since one cannot "see" the traffic from the other directly, but the AP must "see" both).
The next reduction in bandwidth (for either "a" or "g") is signal strength and (IMO, more importantly) signal *quality*.
As the signal strength and/or quality decreases, the AP and client "downshift" to the next lowest (or most stable / reliable) signaling rate. Lower signaling rates *tend* to get longer distances, and the lower signaling rate reduces the affect of most interference.
The encryption will not present any significant impact to the throughput; the hardware generally uses an encryption co-processor to handle that load. There may be a slight / minimal / negligible / insignificant additional latency, but it is fairly constant (i.e., does not significantly contribute to "jitter").
All that being said, and given that you are dealing with shared bandwidth (either band), the next biggest factor will be the type of data you are passing, how much of what kind of data each individual client is passing, and the signal strength/ signal quality (SS/SQ) that is seen by each individual client.
If you have one client, fairly close to the AP, and he's running heavy traffic, clients that are farther away,with poorer SS/SQ, trying to do other high-density traffic at the same time will probably see much poorer performance (if they can even stay connected).
There are some mechanisms you can tweak to make the system a little "fairer" but the general rule is that "louder" stations will tend to win any contention.
Not to sound like a broken record, but the best (and only), most reliable way to accurately determine "real" throughput is a rock-solid site survey.
The site survey will also catch environmental issues (wall surfaces, room geometry, RF densities, reflection potential (good and bad), biological factors (plants, decorative trees, people walking through the RF field, etc.) ....and all of the other zillion thing that will affect your real performance.
Rule of thumb for *EARLY* planning: 22 Mbps per band, divided by the anticipated client load, minus a few percent for "fudge factor" rounded down to the next whole number.
Don't set the budget until you have completed a solid site survey.
Use of the proper antennas can greatly enhance the security and performance of the system.
There's no such thing as affirmitively "obvious" when dealing with wireless systems. Given a choice, assume that it won't work and you'll need to "work around" (i.e., "Line Of Sight" doesn't always work, surprise construction materials, the neighboring school's microwave system blasting in through your windows ....)
Hope this helps you understand some of the wrinkles ... wireless is not quite as cut-and-dried as wired systems.
Good Luck
Scott
