If you haven't seen it yet, Magento and Rackspace teamed up to create a white paper on performance tuning Magento. It's excellent.
https://support.rackspace.com/whitepapers/building-secure-scalable-and-highly-available-magento-stores-powered-by-rackspace-solutions/
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Another great resource, newly available (Oct 2011) is:
http://www.sessiondigital.com/assets/Uploads/Mag-Perf-WP-final.pdf
(Thanks due to Alan Storm on this one.)
