As mentioned in this question/comments there are certain disadvantages when moving from 32 bit .NET to 64 bit .NET.
Probably the biggest advantage is the much bigger process address space in the 64 bit world, but what other pros and cons are worth noting?
I've seen it to be noticeably faster (~ 4x in my experience) for some computationally-heavy (number-crunching) applications. The best thing is it comes for free in pure managed cases. You don't even have to recompile anything to get the benefits. Also, I've heard that the x64 JIT has more aggressive optimizations.
The biggest disadvantage is probably not being able to load 32-bit COM components in process.
Your application may or may not run faster. I've seen improvements for some applications but not others. It depends on how much your application takes advantage of 64 bit (math) operations and if this offsets the larger data and code that x64 uses and therefore has to load into instruction and data cache before it can be executed.
This is worth a read. It's old (.NET 2.0) but much still applies; pointer sizes, COM interop etc:
Migrating 32-bit Managed Code to 64-bit
It's also worth knowing that even on x64 the CLR has a single object size restriction of 2Gb. This isn't an issue for 99% of scenarios but if you're moving to x64 presumably it's because you might be working with large data sets. See here for more discussion:
Are C# Strings (and other .NET API's) limited to 2GB in size?
So. Unless your application uses data that doesn't fit into 32 bit memory or makes heavy use of 64 bit operations you may not see much, if any improvement.
The other downside is that Visual Studio for x64 applications has some limitations:
Edit and Continue is not available for 64-bit debugging.
You cannot debug in mixed-mode, calls from native code to managed code, or vice versa, in 64-bit code.
See: http://msdn.microsoft.com/en-us/library/ms184681(VS.80).aspx
Note: The 64 bit C++ compilers aren't installed by default. You have to select them during the install.
I also just found this (because I'm optimising a x64 application myself).
"Porting and Optimizing Applications on 64-bit Windows for AMD64 ..."
http://download.microsoft.com/download/5/b/5/5b5bec17-ea71-4653-9539-204a672f11cf/AMD64_PortApp.doc
Has lots of good pointers on compiler switches and the like.
64 bit apps will not always run faster than 32 bit.
The two blog posts below talks about it:
https://blogs.msdn.microsoft.com/rmbyers/2009/06/09/anycpu-exes-are-usually-more-trouble-than-theyre-worth/
Larger pointers means more memory and cache consumption, and the number of bytes of CPU cache available is the same for both 32-bit and 64-bit processes.
http://blogs.msdn.com/ricom/archive/2009/06/10/visual-studio-why-is-there-no-64-bit-version.aspx
A 64 bit address space for the process isn’t going to help you with page faults except in maybe indirect ways, and it will definitely hurt you in direct ways because your data is bigger. In contrast a 64 bit operating system could help you a lot! If you’re running as a 32 bit app on a 64 bit OS then you get all of the 4G address space and all of that could be backed by physical memory (if you have the RAM) even without you using 64 bit pointers yourself.
