Hold on here.
How fast a machine is says just about nothing about how many requests it should be able to handle, because you/we have no clue about how much work each request is.
An empty mysql table will handle thousands of queries on a p1-200, and a dual P3-900 will handle a few queries per minute doing a
join on 5 tables of 60million rows each.
If you run top you'll find that it's your database that is slowing you down.
Include files are hardly ever the problem,
and PHP is quite fast too.
Start gathering knowledge about what is really slowing you down.
Use microtime() to measure how long it takes for each element of your page to be processed. (doing the includes, connecting to the database, running each query etc)
One very simple thing you can do to nibble a few ms off your request time is to use mysql_pconnect() instead of mysql_connect().
Don't mess with your setup intill you know what's causing the 'slowness'
