Does it not have the ability for an airport card. And you know the router has ports for wired computers. As for usb nics, I'm not even sure if any work on the Mac. I suggest just reading the lables on the sides of the boxes to see.
for external wireless, I reocmmend forgetting USB and going with an ethernet bridge like the linksys wet11 or wet54 devicse. belkin I believe also makes a 802.11g ethernet device. They don't require drivers and are OS independent, so there's less chance of problems.