What gacutil.exe should I use?

Question

So, on a Win 2008 R2 Std x64, I have 6 (six) gacutil.exe all different (not counting ones in VS folders), in:

1. c:\Program Files\Microsoft SDKs\Windows\v6.0A\Bin
2. c:\Program Files\Microsoft SDKs\Windows\v6.0A\Bin\x64
3. c:\Program Files (x86)\Microsoft SDKs\Windows\v7.0A\Bin
4. c:\Program Files (x86)\Microsoft SDKs\Windows\v7.0A\Bin\x64
5. c:\Program Files (x86)\Microsoft SDKs\Windows\v7.0A\Bin\NETFX 4.0 Tools
6. c:\Program Files (x86)\Microsoft SDKs\Windows\v7.0A\Bin\NETFX 4.0 Tools\x64

Questions are:

1. Does x64 versions have any difference in behaviour?
2. Should I prefer v7.0A versions other v6.0A?
3. Should I use NETFX 4.0 Tools versions for .NET 4.0 assemblies, and "standard" version for the rest?

Or, if there is some general type article on the subject, I will be grateful.

Update 1. Concerning Hans Passant's answer:

1. gacutil.exe is a .NET Framework Tool part of a "Build, Deployment, and Configuration Tools (.NET Framework)" as now obsolete (!) shfusion.dll was;
2. Indeed v6.0A are from VS2008 install, and can be ignored (1, 2);
3. The 64-bit explanation theory also sounds reasonable. I also suppose that there is most probably an IA64 version of gacutil.exe, so it's like a "tool copy for each platform", it's just happend so that x86 version runs fine everywhere because of WoW64. Minus 2 gacutil version from the list (4, 6);
4. The part about which version to use, seems to be wrong. V4.0 gacutil.exe detects and places assembly according to it's target CLR, and works fine for both .NET 4.0 and earlier version assemblies. So the question is, why leave V2.0 gacutil.exe? My guess would be for the purpose of deployment on environments there .NET4.0 is unavailable.
5. To use or not to use GAC, wasn't in the question. So I leave last paragraph with no comments.

Update 2.

So, to me seems like one can safely use either number 3 (then .NET4.0 is unavailable) or number 5 otherwise for all GAC operations on Windows x86 and x64. And the answers to the questions are:

1. No.
2. Doesn't matter, but using newer versions seems more logical.
3. No, you can always use NETFX 4.0 Tools version (if .NET4.0 is available) for all GAC operations.

Answer 1

Gacutil.exe is a Windows SDK tool, not a .NET framework tool. You got two versions of the SDK on your machine. You got 6.0A from the VS2008 install, 7.0A from the VS2010 install. You also have a 64-bit operating system so you got the 64-bit tools as well. Which doesn't matter for gacutil.exe, not sure why they include it separately. Probably to keep the Visual Studio Command Prompt working, note that you have separate 32-bit and 64-bit versions of it. Which matters a lot for C++ projects. VS2010 allows targeting both CLR version 4 and CLR version 2 so that's 6 versions total.

What does matter a great deal is that you have two GACs on your machine. The GAC for .NET 4 assemblies is stored in c:\windows\microsoft.net\assembly. For earlier versions it is stored in c:\windows\assembly. You must use the proper version of gacutil.exe to get an assembly into the proper GAC. For an assembly that targets .NET 4, you must use the one in Bin\NETFX 4.0 Tools. And for earlier ones, you must use the one in Bin.

Using it not at all is best, assemblies should have the Copy Local property set on a dev machine. The GAC is a deployment detail, you'll get in trouble when your GAC has an assembly that your user's GAC doesn't have.