There's no reason you can't use a combination of almost all of those.
- Your web application could be an asp.net mvc web app
- The silverlight app could be implemented using mvvm
- silverlight/server can use ria services for communication
That leaves whether to use silversprite or not ... that really depends on the kind of game. If your game is going to use lots of menus and text, then you might want to avoid silversprite (which is based on xna). However, if you want a more graphical/interactive experience then silversprite is a great choice as it's basically just writing the game with xna.
And then, you could always port your game to windows, xbox360, windows phone 7, and even iphone (via xnatouch) since silverlight is based on XNA ;-)