I picked up some further evidence justifying the move to Nehalem (5500 series XEON) processors over the previous generation recently at a vendor training event. While it has been a little bit laborious to find comparative information between the previous generation of XEONs (5400's) and the Nehalem generation (5500's), I did get to see and then I found a couple of slides from IBM that compared their servers with price / performance crossovers. They are claiming a raw performance increase of 50% or better in most cases for the same price point on their System X server family. They are not taking into account any of the other advancements of the Nehalem family either. This holds true for the other vendors that I deal with as well. For basic cost reasons alone, I am moving any of my clients to Nehalem family right away.

Attached is the slide from the IBM BladeSystem deck with their comparison, you may need to save the file then open it to view it.