Can I force my Mac to make better use of the inactive memory?
You can think of inactive memory like free memory that's been reserved for a recently closed application, but ready to be consumed by other uses when the 'truly' free memory has been exhausted.  If you have 1 GB of memory (for the sake of making this discussion easier) on a freshly booted system, imagine the OS uses 300 MB and you open an app that uses 200 MB.  You now have a total of 500 MB used, and 500 free.  If you close that app that was using 200 MB, the memory is marked inactive and you'll now have 300 used (OS), 200 inactive, and 500 free.  If you open an app that requires 600 MB, it'll use the free memory then pull from the inactive pool.

You can free inactive memory by opening Terminal.app and typing (without the $):

 $ purge


You can, also configure MySQL to use more memory but if you're not having performance problems I'd leave everything alone:


http://dev.mysql.com/doc/refman/5.0/en/innodb-configuration.html
http://dev.mysql.com/doc/refman/5.0/en/memory-use.html


For more information on inactive memory in OS X:


http://support.apple.com/kb/ht1342 (search for "inactive memory")
http://hints.macworld.com/article.php?story=20010613140025184
http://developer.apple.com/library/mac/#documentation/Performance/Conceptual/ManagingMemory/Articles/AboutMemory.html