Thanks again for all the responses!

The hydronic network website is wonderful; I've bookmarked it, and will use it as a starting point for my research. I like the idea of using the boiler to supply our hot water. We don't place huge demands on our hot water supply, and had been thinking of going with a tankless hot water heater next.

Yes, I imagine that hot air can be quite comfortable if competently installed. My biggest concern is whether it can work effectively in our old, uninsulated-wall house (and yes, I agree with the suggestion that adding more insulation would be money well spent; however, we are not willing to tear up the plaster walls, and I'm a little concerned with the dust factor of blown-in insulation). Anyway, the pro-hot air people I've spoken with typically have newer houses. The people I know with old houses, and our European friends, think we'd be crazy to give up the hot water heat.

Since a new boiler + separate C/A (ducts can go in attic) will probably equal the cost of forced air + C/A, we are seriously considering this option. Yes, we made the living/dining/kitchen area into one large open space, so I will look into the minisplit system. I am not familiar at all with this; but I am wondering if it would be similar to (though possibly more effective) than our current (through-the-wall) AC unit. (?)

Although I like the idea of both the baseboards and radiant panels, I think it's unlikely we will see a return on that investment; so the dollars are more likely to go to CA instead.

ps, I don't know our cost per therm offhand of electricity and gas. Our highest monthly gas bill this winter was about $175; our electricity is typically minimal, around $25-$30. Would we expect to see much higher electric bills with a forced air furnace? (In general; I know this depends on our cost of electricity).

I really appreciate all of your input; it gives us a lot to consider!