Here is a few useful comments by Nick Anderson on the xen-users mailing list.
It depends on your application. Dont forget to look at residual cost of owning a Xeon 5100. I believe that it requires fully buffered dimms each pulling 9 to 14 Watt compared to non-fully buffered dimms for the new Opterons that support DDR2 which I think draw 2-3 watts per dimm. Plus even though the Xeon requires less power for the chip itself they dont take into account the fact that the Opterons have an on die memory controller. The Xeons have a separate memory controller which I believe draws another
Kind of went on a tangent there … but really it does depend on the type of applications you are running. I seem to remember the Xeons whop up on the Opterons in single threaded applications, however I think the reverse is true when you start using threaded applications.
Obviously the figures from the review I mentioned yesterday aren’t so clear cut. However, now Intel are producing solid tech, there is no longer the bad taste from their hiding behind a market position and market power. It will be interesting to see what the Anandtechs and other tech-head reviewers say when they get this equipment. Better choice can only be good.