What speedup can I expect in modern PC games from switching to a 64bit OS?

Forgetting, for the moment, the advantages afforded by more RAM being addressable, what performance gains are typical with say, vista 64 over vista 32?

Do all games perform better, or does it vary widely? Is there a list somewhere of games or game engines that perform a lot better in 64?

Update: Thanks for responses so far.

I guess I'd assumed that by now, most of the major game engines (Unreal, Source, Cryengine, etc) had special versions, not only compiled as 64bit executables but also modified to take advantage of the increased power (I'm imagining some lower-level 3d graphics math or physics or something being faster because of techniques that take advantage of the larger address space and/or any new instructions). Thus some games would have a significant boost in performance. Is this not the case?

From what I gather, 64-bit doesn't necessarily mean better performance, assuming that you compare two identical environment which only differ in their bitness. Ofttimes performance will even be on the 32-bit system's side. This is due to the fact that all pointers on a 64-bit system have a larger memory representation (due to the larger address space) and therefor working with them is slower than with 32-bit pointers (though this effect is always true, not just when games are concerned).

However, you shouldn't just trust what I gather. It is much better to trust what Tom's Hardware gather. Their bottom line is something like: "despite claims from proponents of 64-bit it still looks like no-one is writing their game engines to take advantage of it yet - not in a way that creates significant performance gains".

To get the benefit of running on a 64bit OS the game would have to be compiled and linked to run as "64bit" rather than "32bit", plus there might be modifications needed to the code to make best use of the new features.

You probably won't see an improvement in frame rate or graphics detail as that is pretty much controlled by the graphic card and GPU.

You might benefit from faster level loading times as more (or indeed all) of the level can be kept in memory.

One of the main advantages of switching to a 64 bit OS is the ability to properly use more RAM.

Here has some more info:

What performance gains are typical with say, vista 64 over vista 32?

Your CPU can process larger chunks of data per instruction, if the program is written to take advantage of that.

Now, games typically do a lot more GPU work than CPU work, but still there is some load on the CPU that can potentially run faster on a 64 bit processor, if programs and compilers are optimized to take advantage of these faster opcodes.

If you have more than 4GB or more of RAM in your 64-bit machine, then memory-hungry 32-bit games will benefit, as every 32-bit app gets an entire 32-bit address space (ie roughly 4GB) all to themselves.

Whereas in 32-bit Windows they have to share that 4GB of 32-bit address space with Windows itself, other apps running and various other bits including hardware shadowing (your video card's RAM is normally shadowed into main memory).

This is why many memory intensive desktop apps (like heavy Photoshop use, big projects in Visual Studio, etc) are starting to recommend you run them on 64-bit Windows even though the app itself is only 32-bit.