Howdy, all. Thought I'd take this chance to de-virginize myself with the mp3car forums, and get my $0.02 in to boot.
I've run sw RAID 1 and 5 arrays (separately) on Linux, and writes with RAID 1 are definitely slower, but reads faster, than a single drive on the same interface would be. Can't speak to a performance increase/loss with RAID 5 - just hadn't paid much attention.
I'm in the process of gathering equipment for my car computer (refuse to call it an mp3 box, since I left mp3s behind long ago), and plan to include a RAID 5 array (sw RAID on Debian GNU/Linux, natch) of 3x300 or 400 GB drives, whichever is more cost-effective when I get around to purchasing them. PATA connected through USB 2.0 to the computer. Overkill? Perhaps; but I've been collecting digital audio since '98, and I like to have it all on hand for when I just *have* to hear that one song.
FWIW, after an awful incident with hardware RAID, I don't think I'll ever trust it again.
Software RAID on a decent OS + fast bus + fast CPU = the way to go.
Best of luck to all of you ditching Windoze - you won't regret it!