Simply install the asp.net mvc 3 tools update :
This contains EVERYTHING you need to get started with ASP.NET MVC 3. This will actually even install NuGet 1.2 for you :)
There is NO need to install the asp.net mvc 3 RTM : http://www.microsoft.com/download/en/details.aspx?displaylang=en&id=4211 although if you already have it installed it will not get in the way. The tools update contains the same binaries and the only improvements are in Visual Studio between the two versions. So no need to update projects you have made with the original ASP.NET MVC 3 RTM release.
Compatibility with SP1 / without
It does not matter if you have SP1 installed or not. The special case is when you have Async CTP installed.
Compatibiliy with Async CTP
It is not compatible with the original Async CTP. If you want to use async first install the old async CTP. Then you need to upgrade your Visual studio 2010 to SP1 and then install Async CTP SP1 Refresh. Finally you can install ASP.NET MVC 3 tools update and everything will work fine :)