There is no reason why you cannot use a combination of almost all of them.
- Your web application may be an asp.net mvc web application
- Silverlight application can be implemented using mvvm
- Silverlight / server can use ria services to communicate
Whether it leaves to use silversprite or not ... it really depends on the type of game. If your game uses a lot of menus and text, then you might need to avoid using silversprite (xna based). However, if you need a more graphic / interactive experience, then silversprite is a great choice, as it basically just writes the game using xna.
And then you can always transfer your game to windows, xbox360, windows phone 7 and even iphone (via xnatouch ) since silverlight is based on XNA; -)
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