MySQL replication has worked well for me. i.e. use the replicated version of the database rather than the overloaded primary version.
MySQL 4.x offers query caching and that has worked *really* well for me on a site with few updates and lots of reads.
Update:Gah! Just noticed your paragraph about not changing MySQL version - which renders my comment above pointless.

In reply to Re: Best way to do caching of Class::DBI objects from a MySQL database by zebedee
in thread Best way to do caching of Class::DBI objects from a MySQL database by eXile

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