The 'gargantuan long building on St. James' is still there. It is called the Park Square Building, and contains mostly offices with some retail.
Of those three free-standing buildings between Clarendon and Berkeley, the one not built for MIT is still there. It was originally the Museum of Natural History, which is now (in greatly changed form and location) the Museum of Science. The building contained a Bonwit Teller store in the 1970s, and now contains Louis-Boston.
The other two (MIT) buildings were torn down and replaced by the New England Mutual Life Insurance building (now called 'The Newbry').
I think the 'building with the steeple across the street' is the First Corps of Cadets armory, still standing today as the Park Plaza Castle.
I have no idea about the round building. Some sort of storage tank, maybe?