Month-long trip to Europe...some questions and advice?

My husband and I are wanting to take a month-long trip to Europe next month after I graduate. We want a round trip ticket to London, and to buy a rail p so we can explore all the other countries while getting to see the countryside from the ground as opposed to the air. We would like to see Paris, Rome, Venice, Amsterdam, London, Dublin, Athens, and all the other big cultural meccas. Am I missing anywhere else interesting that is a must-see? How much does a rail p cost that would give us unlimited travel throughout Europe? Any input would be awesome.