Ok STOP. The primary reason to use a NT x64 kernel OS (Windows XP x64 / Vista x 64 / Windows 7 x64) has nothing to do with address space, although that is a good side benefit.
The #1 biggest reason is that the NT x64 kernel was built from the ground up, its completely 100% done from scratch. The NT x86 kernel has much code left laying around from the Win 98 / 2K / XP era, even a few 16-bit DLL's are in there. This is a requirement for the NT x86 kernel to be backwards compatible with software made for previous windows OS's, both 16 and 32 bit. This legacy code and support for undocumented features is what lead to the famous "windlblowz" and blue screen nightmares. It also led to the huge security flaws because the older NT kernels allowed applications access to kernel space memory.
The NT x64 kernel was built from scratch with a better security model in mind, sand-boxing of kernel vs user processes and preventing user mode programs from accessing kernel memory or otherwise interfering with kernel mode drivers. A NT x64 system is very hard to crash from an application point of view. The application can crash but it won't take the system down with it. A buggy x64 driver can still cause a BSoD or unstable behavior, but that is why MS went nuts with requiring signed kernel drivers and such. Everything in kernel mode must be x64 code, you can't naively load a 32 bit piece of code into the NT x64 kernel, it just won't work. Because of that lots of older HW drivers don't work in Windows 7 and there isn't much you can do about it. They sacrificed backwards compatibility for system stability.
32-bit applications are executed inside a virtualized system environment. There is a system component called Windows on Windows 64 (WoW64) that creates a virtual environment to execute the 32 bit code in. This includes 32-bit DLL's for DirectX, a different program files folder, and a completely different registry. 32-bit programs are rooted in the WoW64Node section of the registry instead of the main software section. If a 32-bit program crash's all it can do is take down its own bubble, all other programs remain unharmed.
-That- is the reason you use Windows x64. Not for the memory addressing (2gb application space vs near infinite) but for the system stability and security.