If you do not know what is platform specific assemblies, please read this post:
Assume your platform specific assemblies will be shipped both in 32 bit and 64 bit OSes, you will need to test your assemblies at least in the following combinations:
1. 32 bit assemblies in 32 bit OS
2. 64 bit assemblies in 64 bit OS
3. 32 bit assemblies in 64 bit OS
The first two are easy. The last one is a little bit tricky. Remember start from .Net framework 2.0 beta2, portable assemblies will be running as 64 bit processes in 64 bit OS. If you compile your test applications as portable assemblies, they will run as 64 bit processes in 64 bit OS. That means they will not load your 32 bit assemblies!!!
You need to compile your test applications as platform specific, if you want to test 32 bit assemblies in 64 bit OS.