I was plodding along with the combination of PHP 5.2/FastCGI, and ascertained that 5.3 or even 5.4 might gain me some performance. It cut my RAM use by a third -- but Live Comment Preview hasn't worked since. (I've done two uninstall/reinstall cycles.) A brief switch back to 5.2 did not help.
Site: http://www.dustbury.com/ (yes, I know this theme is old, but the plugin was working with it for the last several years)