Thanks Hatter and Kujako
A bit more information - I'm running a happy and well maintained apple wireless network using an Airport Extreme Base station with two Airort Express extensions.
The Base station sits on my Mac Pro and they're usually connected via LAN, however, I need to move the base station to another room and start to connect the MAc Pro via wireless. To that end, I bought a wireless card, I think it's the same one you've referenced here http://www.bluevoices.com/computers/apple-airport-wireless-card-for-mac-pro-netw ork-adapter-802-11b-802-11a-802-11g-802-11n-draft/.
In network preferences the airport option dipslays but shows as yellow. I can sometimes get my network to show in the available networks (but not consistently) and when I try to enter the password I either get an invalid password message or a timeout message.
It's also worth noting that I've got various other devices connected to the network with no problems.
Any ideas?