US EIRP is 4W for 802.11b, so you can probably run at full power with an antenna having 12 dB gain.
Much of the rest of the world is 100 mW EIRP, with Japan being at a slightly odd 10 mW/MHz & China being a flat 10 mW EIRP. In Japan, that means you can only use 50 mW even at 0 dB gain. In China, unless things have changed recently, you can't use any antenna above 3 dB gain. Dunno about Australia. Numbers are different for 11a, especially in the US.