I have one furnace in my 5000 square foot houswe and want install an additional unit for the upstairs, separating the house into a split system. How do I go about calculating for this? I want gas heat and AC in the attic to supply the entire second floor. Thanks

It's not as easy as it seems. This should answer your question. HVAC Software, HVAC-Calc for Heat Loss, Heat Load Calculations

What your looking for is called "Manual J" calculations.
"Manual D" calculations are use for duct design.