2 Apache Boxes That Back Each Other Up

I have a managed box with rackspace (green) On this I am running a couple of distance learning sites e.g howtofish.com (moodle on apache) and video streaming (lighttpd). This is all working fine.

I'm now looking at moving our "normal" website (drupal on apache) to a managed server with rackspace. (call this blue)

The 2 different parts will be running on 2 different machines, but I'd like to be able to move sites/applications between them if needs be due to load/work/failure

For example if the normal website server goes down, temporarily move that on to the distance learning/media server or vice versa.
Or if after a while we get more traffic on the distance learning server than the website, move the media server over to the normal website server.

Hope that makes sense - I'm not looking at a proper cluster here, but 2 servers I can juggle things between if needs be.

The way I'm thinking of doing it is having both machines with the same setup (apache port 80 and lighttpd port 81 for streaming)
And use Vhosts on both machines setup for all the sites.
i.e. green.flyfishing.com and blue.flyishing.com each have vhosts setup for flyfishing.com, howtofish.com and fishingvideo.com.

This way, whichever server the dns for flyfishing.com points to goes there, and the same for other services.

Then do a nightly backup of the mysql databases to the other server and keep files rsynced.

Then if a server goes down or I want to move x from green to blue I just load the backup of green database for the relevant application on the blue server and point the dns entry for the app to blue.