So then what's the difference between a $15 USB one and a $90 USB one?
The $90 one is probably using wireless AC which is a new wireless standard and it's very fast although you have to have a wireless modem that supports it (and they're quite expensive/not too common at the moment). Either USB or pci-e is fine. I have a $17 usb one from tp-link that has not dropped a 5 bar signal since I plugged it in and I had a ~$30 one again from tp-link although it hovered around 3-4 bars consistently. No difference in speed either. If anything, the usb one is a little faster.
Volcanic Islands is coming out in October, so if you can afford to wait 2 months from now, the 9970 will be more solid than anything currently out, including the 780, while probably undercutting the 780.