I cloned my N800's OS 2008 easily using Clone OS to SD Deb!! Boot from SD made EASY!! then installed KDE via KDE358 PBv3 Codename ZELDA - DEB (for boot from SD), both from penguinbait and both very easy to do.

You still boot into OS 2008 on the SD card (though there's now a deb package to allow booting directly into KDE). KDE is a icon in the Application menu that you can launch like any other Maemo Linux application. Once KDE is running, you will have access to a whole lot of new KDE applications and still be able to run the applications installed in OS 2008. I love this setup sooo much!

Sorry about being screenshot crazy.