K64Enabler lets you enable / disable the 64-bit Kernel of Mac OS X 10.6 Snow Leopard. By the default the 64-bit Kernel is only enabled on server configurations. Note that regardless of which kernel is used, Mac OS X can run 64-bit and 32-bit applications concurrently.

Benefits of K64
• 250% faster system call entry
• 70% faster user/kernel memory copy
 
Drawback of K64:
• The 64-bit Kernel can’t load 32-bit Kernel extensions. Therefore third-party software using kernel extensions that are not yet 64-bit won’t run correctly (you should contact the developers and ask for an update).
• The good news is that most software does not use kernel extensions.
 
To sum up, you should use the 64-bit Kernel unless you need to run incompatible software.

What are the K64 capable machines?:
The K64 capable machines are listed in this Apple Knowledge Base

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