Hi Jim ,
1200 is modular chassis , When you order 1200 AP , there is 2 way to order , one is preconfigured chassis or order chassis and corresponding radio cards .
When you buy 1200 , if you want 802.11a and 802.11b both support you need to buy both radios .
1200 AP is upgradable to 802.11g also .
On 1200 you can have vxworks base or IOS bases .
AP100 is purel IOS based and only supports 802.11b
Ap1100 does not support 802.11a but it is 802.11g upgradable .
Ap1100 is more economical than Ap1200
AP1100 comes with fixed antenna while in Ap1200 you
can remove rubber duckie antenna and you can connect external antenna .
http://www.cisco.com/en/US/products/hw/wireless/ps430/prod_technical_reference09186a00800c9435.html
For clients adapter it depends.
Cisco client does wep on hardware so it is faster .
Also we have ACU for different OS platforms .
Wireless is based on 11meg shared media like wired ethernet
On theory you can have more than 8000 clients on Ap but as you add on more users per AP , per user throughput will drop .
Rule of thumb is use 25-30 users per AP .