Pass parameters to Drupal to Return XML data
Exposing Views as XML is one of the features of the Views Bonus Pack module.  You would lose your idea of using GET parameters, but you can easily build up other paths, pipe in your parameters, and get output as XML.

This is also something that the Services module can handle, but isn't as tightly coupled with views.  This module is very well documented, though.  In particular, I suggest reading the REST examples.  This is an add-on, Services Views, that may fit your needs, but I have never tried it.