Yesterday I got GPS working on my Car PC. I'm using Mapopolis with the trial maps. It works, but it doesn't work perfectly with Landscape mode - hard as it tries.
There's a new version of TomTom Navigator (version 5) coming out soon, which claims to support the hx4700 (v4 did not). So I may check that one out if they have a trial version. Though, I think it might be out of my price range.
In which case, Mapopolis should work fine. It's $99 for all the North American maps. Not bad. And it seems to have no problem playing music with WMP in the background while it's running, which I'm happy about.
I promise I'll have pics up soon...