You want to prepare the most exciting game using as much GPU acceleration as possible. However, is the game going to take advantage of GPU acceleration in Silverlight 3? As previously explained, that depends on the operations performed over the UI elements. If you have any doubts, it is possible to change a parameter and check whether Silverlight is sending the cache bitmap to the GPU for certain UI elements:
SilverlightMonsterTestPage.aspx(double-click on it in the Solution Explorer, found under the
<object data="data:application/x-silverlight-2," ...