The wifi card is easy enough to physically replace, the problem is the unit only has a couple embedded card drivers with no way to add to them. I seem to recall some success replacing with other 802.11b cards, but nothing with g.

All "G" routers should work with "B" cards, however typically you have to set it to a "mixed" mode which can impact maximum "G" speed. I remember when I bought my last router I did so as tests showed it had the best "G" performance when there were also "B" cards on the network.