I would put the infrastructure in place to be able to scale later with minimal downtime.
Depending on what data you are storing I would suggest looking in Cassandra:
The Apache Cassandra Project as this seems to be a lot more efficient that using SQL for storing certain information.
Also you may want to ditch Apache and look into nginx:
nginx
Cache as you much as you can before PHP reads it, use a faster more efficient language like C or Java to do the heavy lifting and leave Ruby/Python or PHP to be the front-end languages which basically just render cached and preprocessed data.