Force Snow Leopard to Use 64 Bit Kernel
By default, Mac OS X Snow Leopard boots using a 32 bit kernel. You can force Snow Leopard 10.6 to use the 64 bit kernel by holding down the “6” and “4” keys during system boot. Likewise, you can switch back to the 32 bit kernel by holding down “3” and “2” during system boot. It may be stating the obvious, but the 64 bit kernel will not work on a machine that is not 64 bit. You can verify the 64 bit kernel by typing
uname -a in the Terminal.
Note: some users suggest this only works in the Mac OS X Server edition, I am unable to test this specifically.