My Asus M2400N laptop came with an integrated Intel PRO/Wireless 2100 mini-pci card for wi-fi access. It was ok initially, but when I upgraded the rest of my gear to 802.11g and 54Mbps, it was too slow. I used an Atheros Cardbus card for a while, using the madwifi driver, but the laptop has only one PCMCIA slot, and it was a bit of a pain to have it always occupied.
Now I have bought an Intel PRO/Wireless 2200BG mini-pci card, which I managed to insert into the laptop with not too much difficulty.
The short story is that it can be done upgrading the builtin wireless card in the laptop, and it works. The long story, with photos of each step, is in the extended copy below.