I don't think that you can swap in an airport extreme card (ie 802.11g), this mac was designed for the original airport card (802.11b). anyways, doesn't make sense since the standard is now 802.11n. the better route is to connect the ethernet port to a wireless bridge (eg Dlink DAP 1360).
i've maxed out the RAM to 1GB (using a part from a burn out ibook :) .
as for the HDD, I've swapped in a SSD of IDE kind, with the appropriate dongle adapter. This is the biggest upgrade in terms of speed, highly recommended. i got a 48GB drive rather cheaply (forgot the details now).
Stuck with Tiger since the graphics card on the machine isn't very strong. boot up time rivals recent MBPs. for casual internet (no flash, no video), it's quite good.
all of this for a machine design that is timeless!!!!