I have 2 zebra finches, 1 being agressive, i've separated them and now i don't know what to do.?

My male finch has been pecking and ousting my female. the feathers around her head and neck have all been plucked and some above her tail. I read in a book that he was being aggressive and i should separate them. I separated them last night. My male seems very sad and my female happy. I don't know what to do b/c i don't want to keep them separated or alone b/c i don't want them to be lonely. Should i get a much larger cage and add more birds? do i get another male and put it with the male and a female to put with the female, or do i add another bird species? help :(