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