Some of the developers are asking, is that possible to use the Visual Studio 2005 to build 64-bit application.
The answer is yes. But not every version of Visual Studio 2005.
To build application target on Itanium platform, you can use Visual Studio Team Suite. To build application target on x64 platform, you can use Visual Studio Standard Edition, Visual Studio Professional Edition and Visual Studio Team Suite.
See the product comparison: http://msdn2.microsoft.com/en-us/vstudio/aa700921.aspx 

Visual Studio 2005 enables you to set up your applications to target different platforms, including 64-bit platforms. You also have the option to compile your application and specify that the application should run on a 64-bit operating system either as a native application or under WOW64. WOW64 is a compatibility environment provided by the operating system that allows a 32-bit application to run on a Windows 64-bit operating system.
Learn more WOW64 from this blog: http://blogs.msdn.com/craigmcmurtry/archive/2004/12/14/301155.aspx

Visual Studio provides 64-bit versions of select components as well as tools specific to 64-bit development.
- 64-Bit .NET Framework
- 64-Bit Visual C++ Tools
- 64-Bit Debugging
http://msdn2.microsoft.com/en-us/library/ms246588(VS.80).aspx 

To configure your projects target on 64-bit platform, you can utilize the configuration manager on visual studio.
Here are the steps:
1. Go to Build menu
2. Select Configuration Manager
3. In the Active solution platform list, select the 64-bit platform for the solution to target.
4. If you do not see the option, select New
5. In the New Solution Platform dialog box, select the 64-bit platform for the solution to target.
6. Now the build of the project will be optimized for 64-bit platforms.

Learn more on 64-bit application development: http://msdn2.microsoft.com/en-us/library/ms241066(VS.80).aspx