My local Best Buy (Folsom, CA) is offering a Zenith LCD screen (for either XBOX or Gamecube) for $50 and including any XBOX or Gamecube title for free. Not sure if other stores are offering this or not.
If you're in the market for an XBOX or GC game anyhow, it's a great deal.
I picked up the GC unit. It comes with a car power adapter for the GC, and also has an AUX video input and switch, so no need to hack your own input.