I'd personally go with the Alfa AWUS036H as suggested below:
http://www.ethicalhacker.net/component/option,com_smf/Itemid,54/topic,2649.0/

I managed to purchase it a few months and the range on this bad boy is mad bitchen.

I ve used the Proxim Orinoco Gold b/g cards with the external pigtail connection which was good for its time. But I totally recommend the Ubiquiti SRC 300mW 802.11 a/b/g MMCX. Its a little pricey but none the less I believe the best card I ve used so far.

www.ubnt.com

Thanks

Not to say that wlan pen-testing is a dead subject however the enterprise-class wireless manufacturers have already migrated to 802.11n (draft-n) over G-networks.  For the most part, b-band is rarely seen because of the security implications.

Having said that, more 802.11n enterprise-class APs are integrating security features to provide an incredible amount of security that wasn't seen in the past two years.  You have APs that do:

- onboard Layer 1/2 IPS
- integration with Layer 3-7 network IPS
- AAA backend authentication
- integration with Network Admission Control (NAC) for posture assessment and compliance
- rogue AP detection
- management frame protection (beacon frame integrity)
- AES encryption to the AP (client association)
- and much, much more

Many customers are buying APs solely for their ability to detect (and mitigate) rogue APs either by careless individuals violating the security policy or by malicious attackers.  Now wireless network analysis is a growing field with tons of potential and part of the new CCIE Wireless roadmap.