The PC card will not be an antenna. Rather, it will be a whole new network port. You will see this in the Network prefpane. Basically, you will be bypassing the internal Airport Extreme card in favor of the new card which just happens to have an external antenna.
BTW, what wireless router are you using. Personally, I do no think the issue lies with the Powerbook, but rather the Airport Extreme base stations. I have an antenna hooked up to mine and am only getting 3 bars of signal from 7 feet. Meanwhile, I am picking up my neighbor's linksys from across the street and am getting 2 bars of signal from his set up. Thinking about replacing my base stations with a linksys anyways after I read one can install a mini linux system on it.