You can get 8-core Xeons (X7550 to pick a random one) and 16-core Opterons (6378 for example). More importantly 2013 Xeons do much more per clock cycle than 20

You can get 8-core Xeons (X7550 to pick a random one) and 16-core Opterons (6378 for example). More importantly 2013 Xeons do much more per clock cycle than 2006 Xeons (and 2013 Opterons for that matter). Improvements in pipelining, branch prediction, caches, etc are harder to nail down to one number like clock speed or core count but can be huge contributors to real-world performance.