Some things to consider:
you may wish to keep hot water radiant heat as it is cleaner, quieter, gentler on your internal environment. Gas or oil fired can make a difference in cost. both in fuel and any hook up fees (as noted earlier).
1. You can have AC retrofitted only, and if so, HVAC usually uses smaller ducts for retrofit AC (ie less cost)
2. You can save some $$ if both AC and forced air installed at same time--maybe (as noted by other post)
3. You may wish to consider adding quality insulation and/or new roof/windows/siding--all of these can dramatically cut energy costs as well as heat loss, and need for AC (and add value to property!)
4. a whole house attic fan can be a suitable cooling solution at lower cost.
5. check with a realtor to determine what neighborhood trends are currently; you can add significant value to your home with HVAC upgrades (maybe 90-100$ cost of the upgrades as value) but you may not as much (perhaps only 25-50% of instal cost added to your home'svalue), depending upon what the trend is in your neighborhood.
NYSERDA has grants and info available to help you with costs.