Just a thought, but have you considered that maybe you can't afford this McMansion you are living in? Perhaps its time to seriously consider an exit strategy. I shutter to think of the bill from oil heat on a place like that, completely and totally unacceptable way to heat that place unless you're Al Gore
If you are willing to do the work, a woodstove will help a lot, personally I'd opt for a freestanding stove and run a liner from it into the existing chimneys. You will get more heat from a freestanding stove. But the inserts are pretty good too. If you are going to buy wood, get it a year in advance, you should buy right now for next year! In the meantime, you can work on building a nice place to store said wood. You are probably going to want more than one wood rack from Lowe's.