Microsoft made available the RC2 release of ASP.NET MVC 3, calling it a true "release candidate". The officially ship of the final ASP.NET MVC 3 "RTM" build is slated release in January. The RC2 release works with both the shipping version of Visual Studio 2010 / Visual Web Developer 2010 Express, as well as the newly released VS 2010 SP1 Beta.
This means that you don't need to install VS 2010 SP1 (or the SP1 beta) in order to use ASP.NET MVC 3.
It can be installed on top of previous release (it should just replace the bits as part of its setup). The one component that willn't be updated by the setup (if you already have it installed) is the "NuGet Package Manager". If you already have NuGet installed, go to Visual Studio Extensions Manager (via the Tools -- > Extensions menu option) and click "Updates" tab. You should see NuGet listed there -- click "Update" button next to it to have VS update the extension to today's release.
If you don't have NuGet installed (and didn't install ASP.NET MVC RC build), then NuGet will be installed as part of your ASP.NET MVC 3 setup, and you don't need to take any additional steps to make it work.
ASP.NET MVC 3 is a framework for developing highly testable and maintainable Web applications by leveraging the Model-View-Controller (MVC) pattern. The framework encourages developers to maintain a clear separation of concerns among the responsibilities of the app -- the UI logic using the view, user-input handling using the controller, and the domain logic using the model. ASP.NET MVC apps are easily testable using techniques such as test-driven development (TDD).