It may or may nor have helped, but I've pushed the max_connections setting for Apache up from 100 to 240, close to MySQL's setting of 250 connections (a few spare are nice). This has pushed the load average up to 15-20 (rather than 4-6), but in my very unscientific testing (click a bunch of links and see how it looks) seems to have improved responsiveness.
-- brion vibber (brion @ pobox.com)