Don't forget a PHP accelerator. I've gotten a x3 performance increase just by turning one on with no other tuning. I would recommend APC or eAccelerator. APC has better support for PHP 5.1.x a better backend interface, as well as being in PECL, which means it should have better long-term support and is less likely to stop being developed. I've noticed on my own site that eAccelerator is a bit faster, though.
I'm curious about the possible performance boost from putting MySQL on a seperate box. In my benchmarks I've never seen mysqld use more than 10% of CPU while Apache/PHP was hogging the rest. I would have figured that, because of this, network bandwidth would be a bigger bottleneck.