There are many options. It is good that you have hot water heat now as you have the most comfortable and economical heat. You only have to be concerned with adding in the best cooling without having to compromise whether you want the better heat or the better cooling.
The air conditioning lesson at http://www.hydronic.net displays the various options to adding cooling to hydronic heat.
You can: Install ductwork from the attic level to take the hot air from the ceiling and introduce cool air at the ceiling. There is a system with 2-inch ducts that can be pulled down to the lower floors through regular walls to cool the lower level.
Install wall or ceiling units that use no ductwork, just small copper tubes from the outdoor unit to the indoor blowers.
A combination of the above.
There is a chiller unit now that can run cooled water to indoor blowers for infinite cooling adjustment.
You can even specify New York type cooling where you turn the thermostat up to 76F for winter heating and down to 70F for summer cooling. (Duh)
The options with independent heating and cooling systems give you maximum flexibility.