Visual Studio 2010 RC is available since February 10th, 2010 for the public. Great improvements were made, especially in performance. But this changes are not available for Silverlight 4 developers because that RC does not support Silverlight 4 applications, only Silverlight 3. If you want to continue working with Silverlight 4, you should stay with Visual Studio 2010 Beta 2 until new Silverlight Runtime/Tools/Toolkit etc is available. BTW, it is not possible to run VS 2010 beta 2 and VS 2010 RC side-by-side.