What's important is your development machine. For example, if you're installing Installmate on a 32-bit computer then you need the 32-bit version of the software. I am running a 64-bit development system so I use the 64-bit version. Installmate will automatically install your software in the appropriate Program Files directory on the user's platform depending upon whether its a 32 or 64-bit program that you've created. You don't need to detect anything. This has never been a problem for us and since our software is consumer oriented (non-power users) I am sure that our end users run both kinds of systems. BTW, we've never had to change registry settings to get our runtime to install properly.
BTW, I have no interest -- financial or otherwise -- in Tarma software and know none of the developers.