Custom post type – filter content by custom meta box content

You can filter your query_posts() results by a custom meta field with the “meta_query” array and its corresponding subfilters. For more information check the WordPress codex on Custom Field Parameters.

http://codex.wordpress.org/Class_Reference/WP_Query#Custom_Field_Parameters

An alternative method would be to setup the new filter variables in WordPress’ query_vars and use that instead of $_GET:

http://codex.wordpress.org/Custom_Queries#Custom_Archives