Upgrading OS X to 10.4.1 makes Mail.app disable all plug-ins. (Like GPGMail).
It can easily be enabled again though (phew), see the GPGMail page for instructions.
Hopefully 10.4.1 will stop the random crashes. According to the release notes they fixed among other things "With this update, Safari no longer unexpectedly quits when Control-clicking (or right-clicking with a multi-button mouse) some kinds of PDFs or graphics within a Safari window."