"Upgrading Debian GNU/Linux from Sarge to Etch"
You really shouldn't run into that many problems. If you have broadband connection 400 packages isn't really that many, I forgot to update my Debian Sid system for a month a while back and had over 800 packages that needed updating.
Upgrade to Etch and leave your sources.list pointing to Etch that way when it becomes Stable you will still be running Stable. Personally I prefer Sid, but daily/weekly dist-upgrading isn't for everyone.