x32 & x64 versions of the SDK

Yes, you understand correctly, even if you are developing in .NET, you must use the appropriate SDK Assemblies. And this was introduced in the latest (to date) SDK version , v.4.0.5.

Here is a brief example on how you must choose the assemblies, by Ross Lotharius

http://rosslotharius.com/archive/2008/08/07/crm-plugins-in-a-64-bit-environment.aspx

If you are not using the right version, this error can occurred

Unhandled Exception: System.BadImageFormatException: Could not load file or asse
mbly 'Microsoft.Crm.SdkTypeProxy, Version=4.0.0.0, Culture=neutral, PublicKeyTok
en=31bf3856ad364e35' or one of its dependencies. An attempt was made to load a p
rogram with an incorrect format.

-f.