Depends completely on your application. A neat setup for a web based app is to have 2..n servers sitting behind a L4 switch for load balancing. The servers need not be amazingly gruny beasts, which is where the beauty lies.. You just add low cost machines when you start to lag at the front end.

If then you need a database, open a nice fat channel from the web servers to a dedicated (suitably tuned) database server on a private network.

Not specifically answering your question, but perhaps a viable alternative?