I use this as a reminder.

Mac OS X Snow Leopard - 64 Bit

This is a strange one. By default, Snow Leopard boots into 32-bit mode. To change this, hold down the numbers 6 and 4 while your Mac boots up. This will tell the kernel to run in 64-bit mode. The downside is the next time you restart your Mac the kernel will revert to the default 32-bit kernel.

To keep this from happening run the following command in Terminal:

sudo systemsetup -setkernelbootarchitecture x86_64

Here is the Apple document.