VMWare, virtualization leader company has changed everything about Enterprise computing.  Recently, I helped upgrades our old ESX 3.5.0 environment to the lasted vSPHERE 5.  A new SAN was in place for that purpose as well.  I can definitely see the difference in performance. It is like driving a 4x4 rock climber truck to driving a Civic.   The upgrade process was relatively simple.  I am not going to have enough space to explain it here but in a nutshell, you first:  install vSPHERE 5 to all your new hosts and connect your hosts to your SAN.  Second, configure the ESX Nics to connect to your existing LAN, and to your dedicated VMOTION LAN.  Third, install the new Virtual Center 5 and hook up all the old and new hosts.  Fourth, all you have to do is vmotion all of your virtual servers to the new hosts!  and the last piece is to do storage vmotion all the old datastores to the new dataStores on the new SAN which is connected to the new hosts.  DONE!