After doing some research, the following USB adapter seems to be working in Linux:
Buffalo WLI-U2-KG54-AI HK$ 270
Surecom EP-9001-G HK$ 210 [RT2500 chipset]
Dlink DWL-G122 HK$ 220 [RT2500 chipset]
Zyxel G220 HK$ 239
I have forgotten the formula for converting mW to dBm, learnt from wireless communication course.
I found a website which provides quite good tools to do the calculation:
Need to compare the output power and antenna gain...