I found a couple of pages which you might find useful.
but keep in mind that the processors you are talking about have different architectures, You’re not going to be able to start a 64bit OS one 32bit host machine– but you could start a 32bit VM on a 64 bit host. Simply put the data/IO path isn’t wide enough and the processor instructions are so different on a 64bit OS that it will not run on a 32bit host machine. Think of it this way— you couldn’t install a 64bit os on a 32 bit P4 system.
To make life easier only cluster ESX hosts which are capable of VMotion, such as all 64bit and all 32, if you have 32 bit machines on 64bit architecture then seperate them and switch off the 64bit enhancements to not confuse yourself. as the guys says architecture is the key however it does not have to stop you if you plan ahead and implement accordingly, use the 32bit hardware for machines that do not migrate such as print servers etc, easy planing easy admin.
hope this helps