Quite useful to know!

1. Choose About This Mac from the Apple () menu in the upper-left menu bar, then click More Info.
2. Open the Hardware section.
3. Locate the Processor Name.
4. Compare your Processor Name to information below to determine whether your Mac has a 32-bit or 64-bit processor.
List of Mac 32-bit and 64-bit processors

Now Determine if your MAC OS is 32-bit or 64-bit capable

1. Type this command into the Terminal: ioreg -l -p IODeviceTree | grep firmware-abi

>> Output: | | “firmware-abi” = < "EFI64"> (Note: In my case, i got 64-bit)

In fact, the new MAC OS X (Snow Leopard) supports 64-bit computing but of-course requires a Mac with a 64-bit processor.

More on MAC OS X Enhancements and Refinements