Booting in Full 64-bit Mode
If you've decided that you'd like to experience Snow Leopard in all its 64-bit glory, then there are two ways of doing so. One method only boots into 32 or 64-bit mode once, while the other is intended to permanently boot into one mode or another.
Boot Into 32 or 64-bit Mode Once
To boot into one of these modes once, hold down their respective keys during your Mac's start-up. So, to boot into 64-bit mode, simply hold down the "6" and "4" keys while your Mac is booting. This will tell the computer to use that kernel version. Conversely, should you wish to try 32-bit mode, you can hold down the "3" and "2" keys while booting. Holding down these keys will only make your Mac boot into its respective mode for that particular instance. Upon restart, OS X will boot into whichever kernel version it had been using.
Boot Into One Mode Permanently
If you've tried the above method and you'd found that there are no problems and you'd like to boot in that mode permanently, you can do so by following these steps. First, go to Applications -> Utilities -> Terminal. Type in "sudo pico /Library/Preferences/SystemConfiguration/com.apple.Boot.plist" (minus the quotation marks). Then you should see a section like this:
What we want to change is the "<string></string>" part. Placing "arch=x86_64" between the tags tells OS X to boot in 64-bit mode. So if we change the string part after Kernel Flags to:
<string>arch=x86_64</string> -- This boots in 64-bit mode.
<string></string> -- This boots in 32-bit mode.
Finally, once you've made the changes, simply press Ctrl-X, type in Y to save the modified buffer, and press enter. Voila! You're now booting using the 64-bit kernel permanently!