Antenna suggesttion

I will purchase antenna for 802.11a & g application. Please give me some suggestions that I need to take notice. Furthermore, if there are any good articles or imformation about antenna, please also tell me. Thanks.

Directional ones pointed at the hub will improve the reception. Even flat ones with 6 dB gain are good. These will also reduce some of the multipath and reject interference from similar systems in the next building as long as the next building is not in the direction to the hub.