I currently design, develop, and maintain a suite of Web applications and utilities that receive ten million hits a day, and my experience has shown that the number one thing you can do to make the biggest difference is to limit the number of times you need to hit the database.
I second Chris on this. No matter how fast your servers are, there is an inherent latency in hitting the database, particularly if you do so continuously and unnecessarily. Caching is usually a good solution to this problem--there are commercial products that will help you (like the Zend Performance Suite), but if you design with caching in mind from the outset, you should be able to obtain excellent results even without them.
Cheers,
Marco
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