I think that more or less any wifi card has x64 drivers. They have limited number of chipsets to choose from and I think that all of them are supported on XP64. If card manufacturer doesn't have it, then chipset manufacturer has it.

I have Canyon CN-WF511, it has ralink chipset and has XP64 drivers. It costs around 15€.

If you don't get XP64 drivers on included CD you can download them from http://www.ralinktech.com/ralink/Home/Support/Windows.html , it's fourth from the top (RT2500 chipset), PCI/mPCI/CB(RT256x/RT266x .