Ok I think I found out where I was going wrong. I put dropshare and PuTTY on N800 and winxp respectively (much better way of doing this job- the N800 can just sit nearby and it seems the tar step runs much faster.) Anyway, I re-installed tar. I also bought a 2gb card for use and just put a DOS partition on the first 150mb (cyl 15000) and used the rest for the N800 filesystem. I ran the tar step twice - it stopped at the !RMT has implausibly old time stamp warning message both times but it looked like everything got copied over.

So now I seem to have a working dual boot system.

This time with tar running in xterm I saw the font files scroll by. Then it booted correctly.

Thanks, fanoush, tama, Milhouse, everybody!