SAPI (Speech API) 5.3 is the underlying speech synthesis and speech reco engine used by Vista. it's Vista only, and is an upgrade from SAPI 5.1 on XP. . SAPI 5.3 is COM based, so .NET developers can still use .NET to create a RCW around it, and use it directly from their .NET apps, ignoring System.Speech altogether.
The COM wrapper does expose some functionality that System.Speech does not. e.g. the SAPI methods can be used to manimpulate which profile is used during recognition (ISpRecognizer.SetRecoProfile), while this is not currently exposed through System.Speech. that said, most .NET 3.0 apps will probably get along just fine with System.Speech.