do a search here for "setback", and you will see the jury is out on whether hydronic systems respond well to setbacks. a high mass system may use a lot of the fuel saved during setback to recover its normal temperature. obviously, for a long weekend, a lower setting will be economical.
your present thermostat wiring may need re-working to give the nest its power. if you make hot water with the boiler, or may do so in the future, the nest is out of its range.
one of the most esteemed pros here is a certified nest expert, and he will no doubt have something to add: http://www.gatewayplumbing.com/