You won't find an server-class engine that also has the desktop interface design tools attached. The big server engines all expect you to use something like C++, C#, Java, or PHP to build your interface.
I, too, would love to see an upgrade tool for access that would spit out some basic C# forms and talks to an equivalent SQL Server database. It seems that would be a big money-maker for Microsoft because they could use it as a way to up-sell customers to a full SQL Server.
IIRC, there might be a way to tell an Access front end to talk to a SQL Server, or change the tables used by an Access front end to really be linked tables into a SQL Server, or something like that, but I've never had to use the feature myself.
