If you’re willing to run Ubuntu 8.10 right now for all 20 nodes, ‘moonshine’ from RailsMachine works really well!
It can automatically build out a full stack (Apache, Passenger, MySQL, etc) on systems that have yet to have been installed (and then continue to maintain and upgrade them).
Rails Machine Projects - Moonshine, Rails Machine Gem, Shadow Puppet
It should work with other distros (and other versions of Ubuntu), but will require some retrofitting.
I’ve used this tool on a few linode instances so far, and have been very pleased (although I’m still learning to use it).
-Kevin