Have you tried RMagick 2.0.0 beta?

The RMagick 2.0.0 beta release has been available for about a month now
(http://rubyforge.org/frs/?group_id=12). If you’ve installed it please
let me know what your experience has been. I need to hear from users who
have had a successful experience before I feel okay about promoting this
to production status. Post a note on the RMagick forum at RubyForge
(http://rubyforge.org/forum/?group_id=12) or drop me a line at rmagick
AT rubyforge DOT org.

RMagick 2.0.0 contains two new features that help monitor and control
memory usage:

  1. The Magick.trace_proc attribute establishes an exit for tracing
    Magick::Image object creation and destruction.
  2. The Magick::Image.destroy! method allows you to selectively destroy
    unused images, freeing the memory they occupy and thereby mitigating the
    need to manually call GC.start.

The Release Notes and Change Log for RMagick 2.0.0 are at