K64Enabler
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.
 
You need a K64 capable machine to enable the 64-bit Kernel.
Download (updated 08.21.10)
K64Enabler is free software. Use at your own risk.
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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.
What are the K64 capable machines?
The K64 capable machines are listed in this Apple Knowledge Base : http://support.apple.com/kb/HT3770
How do I really know I am running the 64-bit Kernel?
You can see which kernel you are using in System Profiler:
  1. 1.Choose About This Mac from the Apple () menu.
  2. 2.Click More Info. 
  3. 3.Select Software in the Contents pane.
  4. 4.Look for "64-bit Kernel and Extensions: Yes (or No)" under the System Software Overview heading.