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I know I can force it to boot into 64-bit kernel by holding down the 6 and 4 keys during the boot process. I also know that some extensions still don't run in 64-bit mode.

So the question is what might I gain by running in 64-bit mode?

If it matters, I have the brand new 2010 15" MBP with the faster processer and 8GB of RAM.

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With a kernel running in 64-bit mode, the kernel might be a little faster - but it's not likely you'll ever notice that.

64-bit applications run in 64-bit mode on OS X (and can access all 8GB of your RAM) even if the kernel is in 32-bit mode, so really, I don't see a point of switching kernels.

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I don't know if it applies to a Mac, but running in 64 bit can apparently require more memory:

The main disadvantage of 64-bit architectures is that relative to 32-bit architectures, the same data occupies more space in memory (due to swollen pointers and possibly other types and alignment padding). This increases the memory requirements of a given process and can have implications for efficient processor cache utilization. Maintaining a partial 32-bit model is one way to handle this and is in general reasonably effective.

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