The different builds of Ghost are for the different platforms used to perform the image operation, not the type of image to deploy. When Ghost deploys a system, it doesn't care at all what the content of the image is, you just need the right version for the OS you're running as a boot environment to do the deploy operation.
So, for instance, the Linux version of Ghost is for running in a Linux boot environment, but it can deploy Windows (either 64-bit or 32-bit) just fine, because Ghost's job is laying down the image content and it doesn't care what the image content is. Similarly, you can deploy a 64-bit OS from a 32-bit Windows PE just fine by running the 32-bit version of Ghost (or the deployed OS can be a version of .Linux).
So, the reason for the failure is that OS you booted to do the deployment is 32-bit Windows PE, not 64-bit Windows PE. If you boot a 32-bit version of Windows, you should run the 32-bit version of Ghost on it.
The reason for including a 64-bit version of Ghost is for running inside 64-bit Windows, which matters in a couple of specific scenarios; the most important of these is that there is hardware using a special boot system called EFI, and on at least some of these machines it's necessary to use 64-bit Windows PE instead of 32-bit Windows PE. The 64-bit version of Windows PE, unlike regular 64-bit Windows, does not include the WOW64 system which lets you run 32-bit executables, on 64-bit Windows PE a native 64-bit build of Ghost is required.
For most users who aren't using this specialized EFI hardware, 32-bit Windows PE runs just fine on 64-bit processors and on that you use the 32-bit build of Ghost to deploy whatever needs deploying.
The second scenario for Ghost64 is for 64-bit Windows Vista, when capturing an image while it's running using Volume Snaphot - the Volume Snapshot APIs are not callable from 32-bit applications on Windows Vista, so you again have to use the native 64-bit binary in this particular special case. However, that's only for capturing the image: once it's captured, an image is an image is an image and you can deploy it with any of the different Ghost builds matches your boot environment.