Product groups urls

how can I change how product group urls are generated? 
I want to change from:
/?product_groups%5B%5D=2-7
to for example product group name + value
/?group-name-group-value
It's only cosmetics I know, but which files are included besides template file?
tim
Hi. The files are:
includes/boxes/box_filter.inc.php
includes/templates/default.catalog/views/box_filter.inc.php
pages/category.inc.php
pages/manufacturer.inc.php