Two servers are better than one?

I have a site with 3,500,000+ hits per day for one php file only (that uses mysql). Is it better to have a bigger box or to have mysql on one server and the rest of the work (images, files, email, etc.) on the other?

Current server is:
P4 3G
RAM: 2GB DDR

Thanks.

 

 

 

 

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