The Apple 4.1.7 and 4.1.8 firmware updates incorporate a number of fixes that dramatically improve system stability and performance. The update also includes a check that validates whether the memory installed in the computer is compatible. This check was added to help alleviate random crashes and stability issues. This memory test disables memory DIMMs that are found to be out of specification, and DIMMs that cannot be determined to be compatible. As a result, some third-party memory that was recognized by previous versions of firmware may no longer be recognized after the updates.
With this set of firmware updates, Apple is strictly enforcing adherence to published RAM specifications. RAM that falls outside of specification can cause stability issues with Apple hardware and software, including Mac OS X. If out-of-spec RAM is installed in a Macintosh computer, the system cannot be expected to perform reliably. Apple has provided RAM specifications to memory developers for some time.
You can read the full article at Appleis Tech Info Library Web site.