Have you looked at refine by taxonomy or taxonomy_list (which I'm going to try to extend, so if it's close to what you need...)
http://drupal.org/project/refine_by_taxo http://drupal.org/project/taxonomy_list
Mostly though I'm not clear on your setup-- "Client" and "Project" are two vocabularies? How then do projects belong to clients?
benjamin, Agaric Design Collective, http://AgaricDesign.com/