Here’s the one-sentence summary of this post:
You don’t need to run the 64-bit kernel on Mac OS X to run 64-bit applications (like After Effects).
Now that I’ve written that sentence, I can spend the rest of this post elaborating on it. But, really, you can stop reading now.
The 32-bit Mac OS kernel can run 64-bit applications, and it can manage ~32GB of RAM. With any current computer, there is no disadvantage to running the 32-bit kernel. Running the 32-bit kernel will not change which applications you can run, and it will not change how much address space your 64-bit applications can have.
The design of Mac OS separated the kernel address space from the client (application) address space, which is why Apple can fully support running a 64-bit program on a 32-bit kernel. On Leopard, this is the only option; and it’s the default on Snow Leopard.
So, to say it again, 64-bit applications will run on Leopard (10.5) and Snow Leopard (10.6) versions of Mac OS X running their 32-bit kernels.
For Windows, you need a 64-bit OS to run 64-bit applications, and you need a 64-bit OS to use more than 4GB of physical RAM in the machine.