When using Windows SIM to create Windows image catalogs, you need to be aware of what system architectures you're going to be deploying Windows onto. Specifically:
-- The x86 version of Windows SIM can be used to create catalogs for x86-based, x64-based, and Itanium-based Windows images.
-- The x64 version of Windows SIM can only be used to create catalogs for x64-based Windows images.
The part where you might get confused however is that both versions (x86 and x64) can be used to read catalogs for any system architecture.
If you plan on deploying only 64-bit versions of Windows 7, it's simplest to use only the x64 version of Windows SIM for both creating and reading catalogs.