Da Penguin makes a very good point that most consumers don't understand. An 802.11g card doesn't do anything for you, speed-wise, if your internet connection is anything but fiber-to-the-home, basically. And g-speeds are only enjoyed at high signal strengths anyway. A g-network throttles back to b-speeds at moderate-to-low signal strength.

The exception, as Da Penguin mentioned, is if you are communicating peer-to-peer. In that case, you're not going through your ISP, so that won't be the speed bottleneck.

If you are just surfing the web, then it won't matter a whit if you're using an older b- or g-card. Go ahead and stick an old Airport card inside that Pismo, and leave the external card slot for something fun, like a vidcap card.