DrayTek have a new guide which goes into explains about aerial coverage and how high-gain aerial works. In particular, it explain how an aerial can be higher gain - the point being that it doesn't magically get more power from 'somewhere', it's always be compromise. As this seems to be a widely misunderstood area, I think it's a great guide to have. It's here:

http://www.draytek.co.uk/support/guides/kb-vigor-wlan-antenna-theory

(For registered UK users only)