Great idea. String together a couple cheap/free db's on your backend and treat them like a cluster for high performance read access (which is what most webapps seem to be anyway, at least in terms of percentage of reads vs. writes).
"C-JDBC provides a flexible architecture that allows you to achieve scalability, high availability and failover with your database tiers. C-JDBC instantiates the concept of RAIDb : Redundant Array of Inexpensive Databases. The database is distributed and replicated among several nodes and C-JDBC load balance the queries between these nodes."