One of the biggest advantages to using a Unix box is that you get to compile your software yourself, and it really does make a big difference to the speed of your software. If you are able to, I suggest you compile Apache, PHP, and MySQL yourself using GCC 3.4.x, and as many optimisations turned on as you have time to wait for.
Particularly important here is the PHP compilation, as you are not likely to get much improvement in your MySQL compilation over the stock binaries you can grab direct from MySQL.
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