Wireless for an old system, I presume. There's lots of PCI and USB wireless network adapters but only for the existing standards...
If you're DESPERATE, there are some USB-to-ISA projects out there that have some products but it's not that cheap (not outrageous either) but it'd still be cheaper to get a new wireless router and a couple wireless NICs to go with it, especially if you can find some discounted 11Mbit units since 44 and 55Mbit units are out there now.
But as suggested, the Chaintech 7AJA2 or the microATX version would both be decent. One ISA on a KT133A chipset for Durons and Thunderbirds. I don't know if it supports any XP processors offhand....