What is the benefit of using a 64-bit computer?
You can access more memory. See this article for more info on memory limits.
Can my computer run a 64-bit version of Windows?
To run a 64-bit version of Windows, your computer must have a 64-bit-capable processor.
Can I upgrade from a 32-bit version of Windows to a 64-bit version of Windows?
No. If you are currently running a 32-bit version of Windows, you can only perform an upgrade to another 32-bit version of Windows. Similarly, if you are running a 64-bit version of Windows Vista, you can only perform an upgrade to another 64-bit version of Windows Vista.
Can I run 32-bit programs on a 64-bit computer?
Well the Microsoft properganda says its no problem - but in my experience, as of Oct 2009, there are a lot of programs out there that won't run properly on Vista 64.
There are a lot of hardware devices that just don't work on Vista 64 - and there is no solution for this (other than have dual boot/operating systems).
If I'm running a 64-bit version of Windows, do I need 64-bit drivers for my devices?
Yes, all hardware devices need 64-bit drivers to work on a 64-bit version of Windows. Drivers designed for 32-bit versions of Windows don't work on computers running 64-bit versions of Windows.