There is no "Emulation" between 32 and 64 bit Windows. It would be, and is, completely unnecessary. In one sentence: x64 includes all the instructions in the x86 spec and it is *all* written to Windows API's.
(In advance - I am grossly over simplifying things for the sake of explaining the concepts. And ask that those more technical than I withhold the flames and instead add their kind wisdom)
Not technically accurate, but I find it easiest to think of it like a sandwich - The CPU on one side, and the operating system on the other.
On the processor side:
The CPU reads the actual binary code, which is presented in the form of instructions. These instructions are the "x86" and "x64" that you read/hear about. Understand that the X64 instruction set includes everything in the x86 - So any x64 processor can and will fully handle anything that's x86.
Therefore, as long as the game was compiled to the x86
(32bit) standard, the CPU can fully understand and run it because they are 'speaking' the same language.
On the side of the OS: There is a similar mechanic, though here it's called an "API"
(Application Programming Interface). In very broad terms, it works like the instructions sent to a CPU: These are the commands and formats programmers use to talk to the Operating System, which they use to access system resources like memory and information on the hard drive. You can think of it like the teller window at the bank: It's your way to pass an instruction inside in order to get the result you want. When you go to the teller
(API), you have to give her a message
(instruction) that she understands, right? The OS wants to see some Function
(Withdrawl), the location required
(Account #), and some data set
(how much). If you give the teller
(OS) that, then you'll get your twenty bucks.
As long as the program
(game, whatever) follows the proper API's then it will run on the Operating System.
So: Just so long as a given
(32 bit) game is written to the proper Windows
(Vista) API's, and compiled to run on an x86 processor, then it *will* run on 64 bit
(Vista).
Generally speaking, when you hear about incompatibilities it's because the programmers who wrote a given application either did not adhere to the proper API spec when they wrote their code, or because they took short cuts
(which may no longer work), or because the (new) Operating System's API set is different from the old one.
IN PRACTICE - There is little difference to the user between 32 and 64 bit Windows. Certain aspects may be a few percent weighted one way or the other, but from an end user perspective they look/feel/play pretty much the same. WOW64 doesn't 'emulate' - it plays a few games with pointers and registry entries in order to smooth the coexistence. That is all.
{Edit} If you want/need deeper technical knowledge of the inner workings, then I highly suggest that Tom's Hardware, like most internet message boards, is not the proper place to find accurate information. Rather, you should use TechNet and MSDN to confirm Windows related detailia, and buy/obtain the actual instructional and educational manuals for both general and windows specific theory. - You don't need to pay a subscription fee to use the boards there.
{EDIT part Deux}
http://msdn.microsoft.com/en-us/library/aa384219(VS.85).aspx
On the x64 processor, instructions are executed natively by the micro-architecture. Therefore, execution speed under WOW64 on x64 is similar to its speed under 32-bit Windows.
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