Uh, do you mean a USB adapter you can plug in to be able to connect to a Wireless network, or like a USB adapter you plug in to be able to connect anywhere, like in a car or something?
so I can convert my desktop to receive wifi
So you want a USb adapter you plug in to add wifi capabilities to your desktophttp://www.amazon.com/s/ref=nb_sb_ss_c_0_20?url=search-alias%3Dcomputers&field-keywords=wireless+usb+adapter&sprefix=wireless+usb+adapter%2Caps%2C143
You only need ethernet if you're willing to always have a cord to your computer
yes
As well as lower ping, better stability, and if you're transferring files via LAN, or if you've got some insane internet connection, being able to utilize full speed.
How the heck can you transfer file via Ethernet? I never managed to pull it offWould I need a Crossover or a standard cable?