.net – Are there any advantages to compiling an assembly as x64

There's a difference between compiling and targeting.

Compiling the code with the (for example) C# 3.0 compiler will probably give you a boost on performance (very little one anyway) as some optimization for the generated IL code migh have been included. It also allows you to use some of the new features like automatic properties or lambda expressions.

Targeting for a given framework will ensure your assembly works for that framework (and posteriors) and will fail if you target for 2.0 and are using a 3.5 library. No performance improvements will be directly related to that unless your substituting a class from one framework with another "fastest" class. For example, targeting .NET 1.1 won't allow you to use generics and therefore you'll have to use the ArrayList which is considerably slower than List (due to boxing and unboxing).

This is a tough question to answer, because ultimately it depends on what .NET 3.5 features you are using. If you are using some of the new libraries, such as LINQ, then yes, you'll need to install the 3.5 runtimes. However, if you are just using some of the new syntatic sugars introduced in 3.5, you may not. The reason for this is that .NET 3.5 is 100% compatible with the 2.0 CLR.