My development environment varies based on the project that I'm working with.  For my latest project I'm creating a Flex media player and though I started using Gedit, I've moved to Eclipse + Flex Builder so as to take advantage of Flex's Builders code completion feature.  Other features that I needed for this project were support for FTP, Bazaar, PHP, and JQuery.  Here are the steps I took to build the IDE for this project. 

1. The backend is PHP so I started with PDT all-in-one package of Eclipse version Galileo (3.5). 

2. For Flex support, I installed the Flex Builder plugin.  The Eclipse plugin from Adobe is set up for Eclipse version 3.3.x, but you can get it to work with the current Eclipse version thanks to this blog post.  I also opted to use the flash 10 debugger rather than the flash 9 debugger, which is what the Flex Builder plugin will install.

3. For Bazaar support,  I installed the Bazaar plugin for Eclipse.

4. For FTP and JQuery support I installed the Aptana plugin.

Enjoy!