VMware just released a very useful, 54-pages free ebook that collects many the principal guidelines to optimize the performance of the new vSphere: Performance Best Practices for VMware vSphere 4.0.
It includes four chapters:
- Hardware for Use with VMware vSphere
- ESX and Virtual Machines
- Guest Operating Systems
- Virtual Infrastructure Management
If you are planning a migration to the new platform this is a highly recommended reading.