Usually you can tailor your antennas to the coverage pattern you need. AP placement, antennas, and power level are best determined pre-install by a good, comprehensive site survey.
It may be possible to change your antennas to higher gain types (i.e., 180 degree patch or sector antennas instead of Omnidirectional rubber ducks).
Be advised though that changing to higher gain antennas will also change their coverage pattern; you might reach farther in one direction but less in another. Each antenna should have a chart that describes the side profile and overhead profile of the radiation pattern.
Running in mixed mode (b & g) will reduce your throughput, but it shouldn't affect your range. The brief version is that it takes time to switch modes, and the AP has to notify clients on one protocol that there's activity happening on the other protocol ... it all takes time & bandwidth.
Both are the same frequencies and have the same basic coverage.
CCX (Cisco Client Extensions) should not affect your client's range or throughput; they are extensions - if you don't use them, they are pretty much ignored.
Each vendor has there own "formula" for when to roam to the next AP. Some vendors use Inter Access Point Protocol (IAPP) to communicate between the old AP and the new AP so the handoff happens in a coordinated fashion. Using a single vendor for all of your APs (at least in the same / adjacent neighborhood) increases your chances of a smooth handoff.
Good Luck
Scott
