I might have agreed with you 3 years ago when gighertz was a the only valid indication of a processor's potential performance. The days of rating a processors solely on it's core speed are long gone. For example, the X2 3800 is "only" 2GHz and it out-performs PentiumD's rated at higher clock speeds; and nowadays, the E6600 @ 2.4GHz bests an X2 4800+ @ 2.4 GHz, so...read
this article, it puts the gigahertz question into perspective.
Performance per watt is a very valid measure of a processors performance, especially in the context of a datacenter and doing everything you can to cut costs, use less energy, and still deliver the performance that the business needs.