In the beginning was the Xeon, and it was 32-bit.
Then Intel moved over the face of the Xeon and created Nocona, which was 64-bit, and Intel thought it was, well, OK.
So Intel took Nocona and added more L2 cache, and thus begat Irwindale. Intel saw Irwindale was good, but Opteron was still better.
So Intel was wrathful and split Irwindale asunder internally, creating Paxville DP, with dual cores.
Intel looked at Paxville DP and said unto itself “still not enough cache!” and soon more cache grew within the Paxville DP and thus begat Tulsa.
Thus endeth the lesson, from the book of Wikipedia, Chapter Xeon..