I have hydronic floor heating in the sunroom in my house. It is wonderful. However, right now it is just running off of the hot water heater. Going forward with my home projects, I'll be using hydronic heat as I would like to go with a solar heating system as well.
From what I have read, 50% of your heating from solar isn't too hard to do. 70% is more difficult, and 90% is really quite high. You'll need a fair amount of panels and also a very large amount of thermal storage to maintain heat during cloudy days/weeks.
In order to size your system, you'll want to estimate the heat loss of the house to see how much energy is required to heat it. I think a very easy way of doing this is looking at previous bills. X therms of gas = X kWh (you can throw in the efficiency loss of your furnace too). You'll compare that to a
solar map that shows you how many kWh of heat you can get from the sun per square meter. It looks like you can get 4-5 kWh. From that, you can calculate the square footage of collector you'll need to heat your home. I'm not sure exactly what kind of efficiency Gary has been getting out of his panels either, but you'll have to include that. You'll also have to take into consideration that you can get sun for X hrs each day, and that you'll also have to harvest more heat for cloudy days, etc.
If you don't think that your house is as well insulated as it could be, I'd really start there (and I am actually). Insulation is cheap compared to solar stuff.