Recently I was envolved with moving a site from one server (VPS) to other. I had to make a VPS installation also and setup of the server. I used Debian 6 which usually comes with php 5.3. Sometimes some modules have problems with 5.3 version of PHP. So when i transfered drupal site(drupal 6) I kept getting 500 error on server and tought this was somehow due to 5.3 php and some module i had, as on original location I had 5.2 so i was trying to force 5.2 to this server and lost bunch of time on this. In the end i finally realised this error was because of the original low PHP memory set on new VPS, which was 32 MB. When I raised this to 256 MB everything started to work even on 5.3.