A second NIC really comes in handy when doing this. I use to do it that way until I realized I could just put my other NIC in the PC and not worry about diconnecting from the internet every time.
Only bad thing is, I have tohave the Xbox on when I start up my PC, or it won't connect to my xbox at all. Once my PC starts, I can turn off my xbox until I need to FTP to it. Anyone know a way to set it up so I don't have to have my xbox running when I start up my PC. It gets annoying when I go to FTP my xbox and realized I forgot to turn it on when I booted my PC. I then have to restart my PC with the xbox on again.