While this book will not provide an exhaustive guide on how to set up the environment for building and deploying OBAs, I can provide you with the baseline system requirements so you can go ahead and set up a sandbox environment where you can build and test your own OBA. These are as follows:
Windows Server 2003.
MOSS 2007.
Visual Studio 2008 (Professional SKU or higher).
Visual Studio Tools for Office (VSTO) 3.0. (Note: VSTO ships in Visual Studio now, so if you have Visual Studio 2008 Professional or higher, you will have VSTO.)
Microsoft Office 2007 Professional.
Office 2007 Programmability Interoperability Assemblies (PIAs), which should install as part of the Visual Studio 2008 installation process.
For your hardware, I would recommend minimum server standards, so at least 2 gigabytes or more of RAM, I gigahertz or more for the processor (preferably dual processor), and at least 60–80 gigabytes of hard-disk space.