I am not sure how complex your data is that you are storing in text files, but if your application is doing any searching or looking up of data , I would strongly suggest switching to some DB engine.
In most of the instances I've seen DB engines will easy outperform any txt file lookups. Plus on top of the performance you get advanced query features of most today's DB engines

The most common move from a single server I've seen is to one MySQL server, and one http server. Of course, you can setup a MySQL cluster too.

You could have one server for the database data, two HTTP servers load balanced and a bunch of 'middle-man' servers processing the database requests in a cluster.

http://dewa03.unep.org/manuals/mysql...mponents-1.png explains it in a easier way than my rambling