VMware has released KB articles outlining the best practices for upgrading to vSphere, installing vSphere and upgrading ESX 3.0 virtual machines to ESX 4.0 hardware.

Upgrading to ESX 4.0 and vCenter 4.0 best practices

Installing ESX 4.0 and vCenter 4.0 best practices

Upgrading an ESX 3.x virtual machine to ESX 4.0