I’ve finished the migration more or less. The great news is that the new configuration has fixed the performance problems and it looks like there is now plenty of headroom for growth.
I now have one server with MySQL and the application server on it which serves the pages of the site. The other server does the photos and outgoing mail. The photos are uploaded to the application server but are accessed via a caching proxy (Apache) on the other server.
At the moment the load on the MySQL/application server is no more than about 0.50 but the photo server is double that. Part of the problem is that I haven’t yet managed to get mod_expire working so the headers are missing.
However I am now pretty happy that this configuration will take me to at least 2 million pageviews a day. It’s also just a huge relief not to have the site on the edge of collapse!
PS. I have also signed up with rsync.net for backup. The Planet do have backup options but rsync.net is a bit cheaper and also I prefer to have the backup with a completely different provider.