That being said, this guide uses very clean methods to install beryl so it shouldn't break your setup. The only part that can (realistically) break your system is the xorg.conf editing, so take a copy.
Read it here
STEP 0: Basic preparation
Use OFFICIAL debian repositories.
Install Debian Etch. Update it.
Set up X.org so it works like you expect it to (right resolution etc).
Install accelerated drivers from nVidia.com.