How difficult is it to improve upon bad grades. Currently, I am getting Cs and Bs, I would like As. Am I trapped in school mode by thinking it should only take one semester to get an A?

I am currently studying a masters in scottish history, but would like to do another masters in a field I am interested in. I have narrowed it down to a MA in either intellectual history or in the classics. I would like advice on which one to choose for a future career as a historian. I think the intellectual history is more appropriate but that the classics would look very good on a cv. I do not know what I plan to do my PHD on. Is it advisable to do both, or can a person have 'too many masters'?