You don't say whether you wish to increase all round coverage or just in a particular direction.
Have a look at this comparison chart to see how the directionallity varies, be it in the horizontal or vertical with gain :-
http://www.solwiseforum.co.uk/downlo...comparison.pdf

Depending on where you live you will have to look at the legal requirements, regarding the EIRP of your system, and as you increase the gain of your antenna you may have to reduce the transmitter output.

A WiFi system does not just rely on your eqpt for communications but also on who you are connecting to, so their installation has an effect on the distance and the Data Rate you are using.

Link Calculators:-
Wireless Network Link Analysis
Link Calculator : Wifi Extreme, Outdoor Wireless Systems