Went with R-38 worth of blown-in cellulose - which "bought" us about ten degrees vs. where we were at before
But, when temps dropped into the teens, the house temperature fell again. When outside hit 10 degrees, the temperature inside hit 58.
An inspector found that our thermostat is entirely decorative - the control board on the boiler is partially broken - we've just been manually controlling it from the board and the heat is fine. The inspector also drained a lot of water out of the system - which was black - to clean that up.
Of course, we're still running with a 34yr old boiler and, come summer, an 18yr old A/C - exploring geothermal costs since both the boiler and the A/C could easily die this year.