I wanted to play with a 64 bit OS once, so I installed VMWare to experiment with it. My box had a 64 bit AMD CPU, and was running a 32 bit linux OS on it. I didn't think I would be able to install a 64 bit VMWare guest OS on it, but it worked fine.
Later I tried this all again but on an intel 32 bit box, and it didn't work.
I just found a link that describes under exactly what conditions 64 bit guest OSes will and won't work:[kb.vmware.com].