Yes, it is possible to build such a site with Sitefinity, as we use standard ASP.NET controls, which makes Flash embedding a breeze. And you will have to implement a service.
If possible I reccomend using Silverlight technology. Being a client-side technology, you have no access to the server. For example, in vanilla Silverlight, the only way to bind a data grid is to build a web service
The standard location for Sitefinity's services is ~Sitefinity\Admin\Services. If you go there, you will see that in Sitefinity, we already use client-side data binding for the Generic Content-based modules.
All the best,
the Telerik team