A major caveat to volunteer work: if it's a choice between taking a class and volunteering at a museum, TAKE THE CLASS. The type of clerical work you will be allowed to do will most likely not result in a useful recommendation letter. A letter from a volunteer supervisor at a museum is completely meaningless for grad school applications (except, possibly, as further corroboration of two stellar letters from faculty). Volunteering will also not produce a faculty-vetted writing sample.
With a good writing sample, a great statement of purpose, at least one current letter of recommendation, and funds to pay tuition (which it sounds like you have) you should be able to get into a good MA program and thence, if you do really well, into funded PhD work.
