Well, PC Card can go up to 8 MB/s, which is much faster than 802.11b, so the card slot isn't the limiting factor if you are only using 802.11b.
Yes, 100 Megabit Ethernet will be limited (as 8 MB/s comes to 64 Mb/s,) but unless your internet connection is faster than 64 Mb/s, you won't notice. (Heck, on the 1400, the speed of the computer itself is likely the speed limiting factor there.