I will be building a 2500 sqr ft house on a lake in South Carolina and will be using geothermal heating and cooling. We will be using one of the new foam insulation  and our hot water will be solar heated for the whole house including an outside  hot tub in the winter. I hear people raving about radiant floor heating, but does  it make sense to use radiant heating and cooling in this situation, seeing there will be so many hot days?

Any other building suggestions?

Thank You

Radiant cooling will not take care of the (very substantial) latent loads seen in the Carolinas, but would help with the sensible loads and the overall comfort. Radiant cooling is much more effective if the cooling panels are walls or ceilings rather than floors (cooled floors will pool the heavier cold air near your feet and won't convect. With walls/ceilings the cooled air will fall, creating convection currents.)

A dehumidication coil in the ventilation (I'm assuming you're planning on active HRV/ERV ventilation, since you'll be in a very tight spray-foamed house) or a separate dehumidification system run as a zone off the geo under dehumidistat (not thermostat) control will both cool & dehumidify. Active ventilation should be ERV (with dessicant-wheel dehumidification) rather than HRV (air exchange only) in your climate zone, to lower the load on the dehumidification system.

Geo radiant heating will be much more comfortable than low-temp air, and radiant cooling is nice too.

(I meant to add...)

But you'll always need some means of dehumidifying the place. (60% RH is about right for comfort and keeping mold under control, 50%RH will keep dust-mite poplulations down, for those with allergies.) Tight houses accumulate humidity, and un-conditioned summertime outdoor air in SC will add, rather than subtract from the humidity burden.

I agree with Dana. Cooling will be a more important consideration than heating. And while I absolutely love the idea of radiant heating, I'm not sure that radiant cooling is currently a viable option in the south. Too much heat and humidity to handle for it to be economically feasible. I suspect that a traditional geo thermal system will keep you quite comfortable in your climate, and with far less complications.