There's a resource called pubmed that you could use to fairly easily answer all of your questions. People have been studying Zn fingers for a while and there are many papers describing the effects of the mutations you're interested in. The domain referred to by Neilfws is one of the most widely known, in which zinc is coordinated by both cysteine and histidine. So to answer your questions, mutation of the coordinating cysteine will disrupt function of the protein. Mutation of the coordinating histidine will disrupt function of the protein. In general, there is less tolerance for Cys ->Gly than for His-> Gly, but this may have no relevance to your question, as there are so many contexts for Zn fingers that it's hard to rationalize a general principle or specific instance from your question. Another resource you might find useful is a search engine called google. You can use it to find a wealth of information on zinc fingers and the effects of amino acid substitution on protein structure (there's a nice attribute called google scholar, which will send you to academic articles). Between that and pubmed you can find answers with a minimal amount of work.