802.11g at 54mb/sec, IE, about 7MB a second, is theoretically pretty well short of saturating an as-fast-as-possible 16 bit PCMCIA slot. (The PCMCIA 2.0 specification allows a clock speed up to 10mhz with zero wait states, making it hypothetically capable of around 20MB/sec. Most computers run it at 8.33mhz, the same as any ISA slots/internal busses they might have.)
That said, they made 10/100 Ethernet cards using 16 bit PCMCIA, and I recall them being somewhat slower in practice than the CardBus versions even though in theory 100mb Ethernet should "only" need around 12MB/s bandwidth. So depending on what overhead there might be with polling the slower bus full-speed 802.11g well be pushing it. (For instance, I remember being amazed at how much better my computer behaved when using a SCSI scanner when I replaced the cheapskate ISA controller included with the scanner with a PCI one. I don't think it actually scanned any faster but the computer wouldn't effectively *freeze* while doing the data transfer anymore.)