Increase the amount of heap used by Eclipse/FlexBuilder by adding the following command line options:
-vmargs -Xms256M -Xmx512M
Shortcut Example: "C:\Program Files\eclipse\eclipse.exe" -vmargs -Xms256M -Xmx512M
Alternatively, you can update your eclipse.ini file.
More information can be found on Ryan Phelan’s site.
These examples have been tested on a Windows XP PC with 2GB RAM.