The server chips that you can currently purchase in a server are as follows:

This list is not inclusive of every processor made and does not mention CPU's such as the Alpha which is essentially defunct now.  The list is roughly in the order of strength.  If we rearranged the list in order of a price/performance it would look more like this:

The primary competition for the Opteron is the Xeon and not the Itanium.  Intel has a healthy profit margin selling this CPU in quantity though some question this venerable processors ability on how it compares to the Opteron.  Both of these processors are available for "build-it-yourself" makers and you can't underestimate the importance of the customer driven, owner-built server solution.  This allows maximum control in the selection of server components and the ability to replace them.  Smaller enterprises often find the control of the server marketplace to hostile for their purposes.  Try building your own Power4, Itanium, UltraSPARC or PA8700 server sometime, the entire chain of products, both hardware, software and service is a monolithic controlled structure that emphatically discourages anyone wanting to build their own hardware solutions.

The Opteron has a long way to go to match the sales of the Xeon, however it is off to a good start.  By server number it already outselling the Itanium.