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I would have to also say PCI went ever possible. I am not a big fan of USB. I find is some cases USB not to be very reliable. Enough so that I avoid it like the plague when every I can.
If you need to boot into Safe Mode with Networking you may find that the USB drivers will not install, limiting an option in an emergency.
I think you will find in the long run the few dollars (about $10 USD) that you will save with USB over PCI will not be worth it.
Belkin makes a very nice card that runs from $40 to $50 at Walmart. I have installed 26 of these cards in the last 18 months without any conflicts or defects.Belkin 802.11g 54Mbps Wireless PCI Adapter