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

You need a K64 capable machine to enable the 64-bit Kernel.

Should I use the 64-bit Kernel?
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 do not use kernel extensions.

To sum up, you should use the 64-bit Kernel unless you need to run incompatible software.