Well since DrJ gave me a plug
I switched to Opteron about 3 years ago. Before I had a dual xeon system. My present system has dual 248s and is capable of having 2 dual core cpus. At work I have Dual Core Xeon 3Ghz or something.
Because some of the engineers that built the Alpha went to AMD before Intel bought the Alpha and I read articles that compared the Opteron to the Alpha. From my use I believe the Opteron out performs the Xeon but I don't have stats or anything like that. Just using both daily for the last 3 years.
As vermaden said get the newer models, I have 5 120mm for case cooling...