I use the Eclipse devlopment environment withPHPeclipse for my PHP / Drupal coding. It is not a lightweight program but it is very powerful and works fine on my computer with 1 GIG of RAM. I like that it connects to CVS and Subversion ... I have my own Subversion site to keep my code "safe" and many open source projects use CVS so right from the IDE you can get the code and create a new project.

There is quite a learning curve but its not too hard. I don't have the internal debugging working yet but ... Eclipse is a very popular JAVA IDE and is super in that environment it that it brings the full deployment process together with plugins for many things.