A good PSU will tell you the combined power available between its 12V rails. An Apevia unit with passive PFC isnt a good PSU. In general you cant just add up the power on the rails to get the total power, a dual 18A PSU is probably only capable of 34A total which is still plenty for a GTS 250.
Single rail vs multiple rail it doesnt make much of a difference, as long as the combined power between the 12V rails is the same the unit will perform the same. Many units that claim multiple 12V rails actually have them all joined together and just claim multiple 12V rails because an old intel spec required it, that requirement was changed to a recommendation several years ago.