Both classes and relative work experience are important. My conversations with several established scholars/professors in art history would imply that unstructured/non-degree online courses are not helpful to strengthen your application. They would, obviously, add to your personal knowledge and may help you discover opportunities such as publishing / participating in a journal. In general, if you are looking for graduate studies in art history, start taking art history classes right now. This field is difficult for break-throughs, and does not easily guarantee a bright financial future. You want to be really serious about your interest before investing time and energy into pursuing graduate studies. You should, at least for your own sake, take several research-heavy art history classes to see if you really like this discipline.