So you can connect to Xbox Live already? If you haven't done so you can use a laptop to share internet with the Xbox 360 instead of having it near your modem or buying that $100 wireless network adapter.
You could always use Windows Media Player or Windows Media center to share files. I use http://tversity.com/download/ TVersity free version. One great thing is it will do real-time transcoding of media files that the 360 doesn't support. During the install just remember to check off the ask toolbar offer.