The main advantage of using a 64 bit machine over a 32 bit machine is that memory is not constrained to 4GB; in a 32 bit machine, the highest address that the CPU understands is around 4GB, but with a 64 bit machine, the address space become much more then this in the order of 2^64.
Now lets talk about running .NET code, code that is 100% managed has no problem when it executes in a 64 bit, this is due to the fact that the CLR takes care of this.
Going back to the basic, when you compile a piece of C# or VB.NET code, the compiler produces MSIL (Microsoft Intermediate Language) code, that contains enough information for the CLR (Common Language Runtime) to start execution, like meta data and the types used and so...
When you start a program the windows loader, peeks at the assembly and sees if the application needs the CLR to be loaded, and the version of CLR to be used and the platform (X64 or X84) the code was meant to run in.
and then the CLR is loaded, the MSIL would be executed, but this code has to be first interpreted (JITTED) into native code according to the current machines instruction set so that the CPU can understand and execute the program.
So, building your managed code in a 64 bit or 32 bit machine produces the same MSIL code, however generally for managed code ,its just when this code gets JITTED you need to worry about machine architecture.
When you install, the .NET framework on a 64 bit machine running a 64 bit OS, the installer installs both the version of the CLR, yes, there are 2 versions of the CLR (2 versions of the JIT as well), one for 32 bit and the other catering for 64 bit architecture. The windows loader will be responsible for choosing which CLR to be used. The loader chooses the CLR based on the information on the assembly the developer sets at compile time; if you note that when you build your .NET application in Visual Studio (2005 and higher), you can specify for which platform your are building your code against, possible values include, 64, 32, itanium and ANY
When you specify X64 for your build platform, then the loader will execute your executable in the X64 bit CLR, meaning that your MSIL code will be JITTED into X64 native code. This is same when you specify X32 for your build target.
By setting the above you are forcibly telling your loader which version of the CLR/JIT to be used, this would become very useful when you are loading some 32 bit DLLs into your application as well, which we will discuss in a few seconds...
When you specify "ALL" for you build target process, the loader will select the CLR/JIT according to the machine your code is running on, i.e if its on a 32 bit, the 32 bit CLR will be used and if its a 64 bit then the 64 bit CLR will be used.
Now lets discuss some important rules...
You cannot run a 32 bit DLLs with 64 bit applications in process or mix 64 bit and 32 bit assemblies in process.
Windows 64 bit OS was designed in such away that 32 bit applications can still run on it...64 bit version of Windows comes with an emulator called WOW64, and all 32 bit applications will be running on this emulator, as per MSDN there is not much of a performance implication on this, more on this here....
Something interesting to note is that, if you install Visual Studio on a 64 bit machine, it would install both the version of the CLR, however, Visual Studio is a 32 bit application, and hence will run on WOW64
So, if you are developing a pure 100% managed application you don't need to worry about porting your code to a 64 bit, an xcopy would just work fine.
However, you might need to review, if your application is...
1) using a third party dlls that are built for 32 bit machines
2) If you are using COM objects
3) using unmanaged code in your application.
4) Serelization of objects, this would be a problem, when you are sharing object state serlized from a 64 bit machine and consumed by a 32 bit machine, this can be overcomed by using XML serelization to an extent.
When porting .NET code from 32 bit to 64 bit, you need to review the above, do necessary changes, deploy and then test it out.
This is an old article that can be used as a guide.