Ok, listen to me.
STOP RIGHT THERE AND DON'T DO IT.
Hot water heating is far far far superior to any idiotic forced air system. Forced Air sucks.
Hot water heating throws a perfect heat and warms the room without giving hot and cold spots like forced air. Also, it helps keep dust down, and stops the spread of infectious disease. It is superior in every way to forced air. New houses all have forced air cause it's cheap. You don't need a plumber to install ducts, anyone with a grade 6 education can install duct work.
The other part of the equation is that your house will not have the proper head room and clearances for air ducts. You will have to do a lot of tearing up to install ducts. This will undoubtedly mean ripping up plaster walls to install ducts. Your contractor will then probably DRYWALL over the wound and you will forever see the discrepency in the wall plaster. Plaster is better than drywall, especially if you live in an area prone to mould. Any hospital of any quality always has hot water heating and plaster walls. Removing the radiators will leave holes in the hard wood (assuming it is authentic and has hard wood) which will be quite unsightly, and impossible to repair without being able to forever see the repairs.
Not to mention that your contractor will probaby choose to merely abandon the old water pipes and you will forever be looking at them.
DON'T DO IT.
These older homes were meant to have hot water, it's part of the charm, it's part of the originality of the home. Originality, or doing renovations in keeping with the spirit of the original design will always add dollars to your investment.
Hot water is also a selling feature in older homes. Those old rads can often be quite ornate.
I live in an old part of Toronto, my house is 1922 but unfortunately, it has forced air. In my area, it's mostly hot water, but some houses were equipped with forced air. (Why i don't know.) If i had hot water, i would not be changing it, thats for sure.
If you want central air, fine, get an attic system. Carrier makes an attic system that can be installed efficiently and easily and you can keep your hot water heating to boot. The cold air will fall from the upper floors to the lower floors quite well.
Trust me, your contractor is selling you a con job. Any new high end home always has radiant heat (in the floors actually) and a separate AC unit.
Don't believe me, go to a reputable HVAC outfit and ask them.
People in the know pay big bucks for radiant heating. Go on google and type "radiant heating" and see for yourself, or here's a link for you.
http://www.wattsradiant.com/homeowner/default.htm
Don't do it man, you will be sorry......
cheers