I recently ran into the situation in which I needed to start a VMware 64-bit guest OS in which I’m running a 32-bit version of Ubuntu (10.10).  After a bit of research, depending on your computer’s processor, you can go into the BIOS and enable the CPU’s “VT” BIOS setting (BIOS > Config > CPU > Intel© Virtual Technology > Enable), which is Disabled by default.  Luckily I have a Lenovo W510 with a Intel Core i7 processor that supports the ability to run a 64-bit guest OS upon a 32-bit host, and all that it took was changing this BIOS setting.  There’s also a utility VMware provides that allows you to check your processor for 64-bit guest OS compatibility:

http://downloads.vmware.com/d/details/processor_check_5_5_dt/dCpiQGhkYmRAZQ==