When VMware introduced vSphere they also introduced a new licensing model based on the amount of memory allocated for a virtual machine when it is powered on. This caused a lot of discussion dubbed "vTax" after which VMware decided to update its Licensing and Pricing terms, this still is causing a lot of discussion for certain scenario’s though.
Some documents to consider reading before updating/installing are:
- Administrator’s Guide
- Installing ESXi 5.0 best practices
- Installing vCenter Server 5.0 best practices
- Upgrading to vCenter Server 5.0 best practices