GUI - Process Interface

[rob]

I have a server-client application. My concern here is strictly with
the client part. The client can basically be divided into 3 parts. Part
1 communicates with the server (which might or might not be on the same
computer). Part 2 does some additional processing of the data coming
from the server. This part also processes the inputs the user makes
through a GUI. These inputs can affect other parts of the GUI which
then have to be updated. The 3rd part is the GUI.

Part 2 offers general functionality that can be used for different
applications. Each application has a different GUI. The current app
uses .NET Windows Forms. Nevertheless, other applications might use
AJAX webpages, etc.

The question now is how do part 1 and part 2 communicate. One way I had
in mind is remoting. But I guess then we would have to restrict the GUI
being implemented to .NET technology. Another solution might be using
XML. My main concern with this is performance. The GUI is really just
doing the GUI stuff plus some sanity checks of the user input.
Therefore, communication between part 2 and the GUI is quite intensive
(must be interactive including mouse events that trigger functions on
the server).

What is the best solution to this problem?
Is XML going to trag down performance considerably compared to if part
2 and 3 are merged?
Any other inputs?

Thanks