Boot Snow Leopard in 64 bit mode by default

 First check if your mac has 64bit EFI open up a terminal window and type the following:

ioreg -l -p IODeviceTree | grep firmware-abi

Your result should be: "EFI32" or "EFI64"
If it is the latter then your good to go.

You can either hold down the '6' and '4' buttons during boot up every time or you can modify the following file to permanently boot in 64bit mode:

/Library/Preferences/SystemConfiguration/com.apple.Boot.plist 

All you need to do is add the "arch=x86_64" as the value for the "Kernel Flags" key and restart your machine.  I recommend using a terminal window to do this as you will need to do a sudo.
I use "vi" so I will do the following:
sudo vi /Library/Preferences/SystemConfiguration/com.apple.Boot.plist 
[if prompted enter your user password]

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
 <key>Kernel</key>
 <string>mach_kernel</string>
 <key>Kernel Flags</key>
 <string>arch=x86_64</string>
</dict>
</plist> 

Save and close the file.

Reboot your machine and your done.