I would have sworn that you could add a external antenna to almost any PCI WiFi adaptor, being they all have the universal external connection.. Guess I was wrong though, as I didn't even find it in the 802.11 article @ THG..
Here is some stuff to improve your AP though! I'll get back to you all with a link to a $30 802.11b card with a ~6 foot external antenna.. I have to search my chat logs, this could be a while!