edit: You do not need to run the 64-bit kernel in order to run 64-bit applications or install more than 4GB of RAM in your Mac.


With the current Snow Leopard seed, only Xserve users get the 64bit kernel and drivers - all other Macs default to 32bit.



If your Mac is on this list, you can force Snow Leopard to boot into 64bit mode by holding down the '6' and '4' keys during the boot process. You can also use the NVRAM or the com.apple.Boot.plist file to more permanently boot into 64bit mode. Holding the '3' and '2' keys will, obviously, boot Snow Leopard in 32bit mode.

Apple is probably limiting proper 64bit support to Xserve machines for now simply because 3rd party drivers are not yet updated (printers, scanner, and so on) for 64bit support.

Only Macintosh machines with a 64bit EFI are able to boot the 64bit Snow Leopard kernel and kexts; this is an artificially implemented limitation by Apple, as 32bit EFI can boot a 64bit kernel just fine.

You can check your machine's EFI by running the following command in the terminal:
ioreg -l -p IODeviceTree | grep firmware-abi
With netkas' pcefiv10.1 bootloader, you can boot into 64bi Snow Leopard even on 32bit EFI Macs.

source [osnews.com]