upgrading w2k3 to w2k8 R2 domain controller

We are in the process of migrating our w2k3 ad domain controllers to 2k8 R2. They are currently physical. We would like to take the virtual route but was looking for the correct process or a step by step. Our VM environment is esx5.1 with 64bit hardware. We have 32bit vm servers running in our environment but I did not know how that would play out with Active Directory. Any information on how to proceed would be appreciated.