Trying for the low-hanging fruit, had an energy audit (no cost to us, paid for by one of those obscure line items on utility bill) a month or so back...highly recommend it if you are at all curious. Still waiting for the results discussion meeting though...
Pointing fieldstone basement walls, spray foaming air leaks in attic, to be followed by some new wiring and more blown-in insulation. !$@#^ spray foam (GreatStuff) and the cans it comes in, if you come within five feet of the stuff you get covered and if you don't use the can up right away it hardens in the valve...at least it does work real well for its intended purpose! Maybe also do 'poor-man's spray foam' of the sill in the basement (extruded foam--blueboard--cut to size and held in by GreatStuff).
lsayre--Windows--don't know what you've got or what your plans might be but, don't necessarily fall for the vinyl guys hook line and sinker. Old wood ones with proper weather-stripping and storms are just as good, and are repairable...GOOD vinyl windows are nice but CHEAPER ones are LOUSY, the thermo seal between panes fails quickly, they're hard to operate ('sticky'), and they don't stop noise too well. And it is possible to negate any infiltration gains with air leaks around frames of replacement windows...we had loads of drafts here (previous owner had replacement vinyl windows put in) until I went round the house with a couple tubes of caulking. Question of $$$ (wood wins esp if you DIY) vs PITA (good vinyl wins) vs aesthetics (wood wins, hands-down).
I've lived with both, prefer wood, but sure understand the time needed to bring em up to par. Strictly comparing vinyl vs wood, double-hung or single-hung. No experience with clad wood, fibreglass or aluminum (other than some real cheapie aluminum sliders in a rental in New Zealand--talk about condensation!!! but at least they wouldn't rot...)