this is what I figured, seeing that both AMD's quad and dual core CPUs and Intel's quad and dual core CPUs still have the older x86 instruction set embedded in them.
Now, can you compile a program (link) on a 64-bit OS, and will that compiled program still work on a 32-bit system?? (Like compiling a Visual Basic.net program on a Vista 64-bit machine and then running the compiled program on an XP 32-bit machine).
The beauty of .NET is that it handles this for you. You can "target" 64 bit in C# and VB, but you don't have to worry about your apps not working on a 32 bit machine. Just use the "Any CPU" default build configuration and you'll be fine.