I use the second option that
The Slayer suggested. I have a wireless card in my tower which is LAN only, allowing my laptop to see my desktop and vice versa, whilst my PC uses the ethernet cable for internet stuff.
I would imagine that any USB dongle that connects to wi-fi will allow you to network with your iPad over the wireless network, although you would need to do some minor tinkering to setup the wireless network on your PC as LAN only, so that you get the benefits of the ethernet port on your PC whilst maintaining the LAN network over the wifi.
If you need any help, just holler, I set this sort of thing up myself not too long ago.