For a simple one-piece solution you could go with one of these:
http://www.gohawking.com/store/produ...roducts_id=140
Or this:
http://www.gohawking.com/store/produ...roducts_id=237
Add a homemade reflector on it to amp up your gain in trouble spots:
http://www.freeantennas.com/projects...te2/index.html
Put your chosen usb device/antenna combo on an extension to place it
in the best line of sight to the Router at the other end. This is a
better solution than a card with a coax cable to external antenna as
the coax loses signal strength and the USB does not.
Putting another reflector on your Router/AP to improve it further.
In fact, if it's an option then first make a reflector for the router
end and see if that improves your reception on the far client end.
Steve