A few folks have asked what is required to develop Web Parts for SharePoint 2007. The article Getting Started with Custom Enterprise Search Web Parts is good read in this regard. In general here is what you need to be aware of.
Setting Up the Development Environment
The supported approach for setting up your Web Part development environment is to develop locally on an Office SharePoint Server 2007 server.
You can use Microsoft Office SharePoint Designer 2007 to import Web Parts to Web pages, but not to create new Web Parts. To develop a custom Web Part assembly, you need to use a development tool such as Microsoft Visual Studio 2005. With Visual Studio 2005, you benefit from a rich development environment and debugging features. For more information about using Visual Studio 2005 for creating Web Parts, see the following:
Be sure to also check-out the new external MOSS developer center located here http://msdn.microsoft.com/office/server/moss/.