I would also like to know the answer to this. My specifics are I'm using a SuperMicro x7sbi based machine. It has 2 gigE ports that I use for LAN and WAN but I was thinking of adding a third port for a Guest wireless network. So performance isn't critical as long as it isn't terrible but it would be a lot easier to use a USB adapter externally than to have to buy the special riser card for PCIe, etc... Any suggestions what would work?
