Intel released comprehensive set of comparison benchmarks for new Xeon and Core 2 processors. Like most, if not all, companies that produce benchmarks they are skewed to favour the new product.
For instance on:
you can see the the old single core Xeon is tested with 16GB of RAM, the 5000 series test server has 32GB RAM but the 5100 series test server has 64GB RAM. The Operteron is tested with only 32GB. That RAM difference is bound to have some very noticable effects.