Depending on your anticipated traffic loads, a high-gain omni in the center with directionals on the periphery aimed at the omni usually works pretty well.
If you are anticipating heavy traffic from each/all peripheral locations, then you may want to go with several APs at the center, each with a directional antenna toward the remote it will serve.
If you are trying to cover a large area for clients (i.e. WISP) then you're looking at a large matrix of APs ala cellular ... conencted either by cabling, or an RF backbone (like the 15000 series ... 802.11b to the clients, 802.11a forms a mesh backbone).
Some details would help.
Let us know
Scott
