Vista64 software

I have looked over all the previous posts about the differences between 64-bit and 32-bit, and it seems like the biggest concern is the hardware. Microsoft requires certified drivers that must be specific for Vista64 so not all hardware will work, but it is straightforward to make sure that all your hardware is fully supported before you upgrade, and it seems if you are building a new machine most new hardware from quality companies now come 64-bit drivers.

But I am still confused about the software. I know that to take advantage of 64-bit,the software must be written specifically for a 64-bit OS. So if your software isn't written for 64-bit running Vista64 gets you no advantage. But Vista64 is a hybrid OS, so it is supposed to be able to run plain old 32-bit software too (through an emulator??, so this means it will run slightly slower than it would in a native 32-bit OS??). Does this mean that ANY software that will run in 32-bit Vista will also run in 64-bit Vista?