All the subfloor specs I will mention here are assuming your floor joists are on 16” centers. If they are 20” – 24” apart you must add another layer of plywood to give enough stiffness for the hardwood plank floor. This is best done by using underlay plywood of at least the 1/2” thickness. And it would behoove you to squiggle some urethane construction adhesive between these layers of plywood to prevent future floor squeaks.
For all plank floor I recommend at least having at least 7/8″ to 1″ thick plywood or softwood board subfloor. This insures the 2″ flooring nails full engagement. A thick diagonal (to the joist) softwood board will make a great nailing surface for the new plank floor. But never lay a new floor in the same direction as the old subfloor boards. Read about the pitfalls of this (it’s free) in this web site. If you find you have a softwood board subfloor, laid at right angles to the joist, you will have to install the 1/2” plywood underlay as a buffer between the two floors. Plank flooring should be always laid at right angles to the joist. This is so the long boards can be screwed right through to the joist, for extra stability.
Never, never, never nail a new hardwood plank floor to a old hardwood strip floor. The flooring nails will crack the old hardwood and you will have a double squeaky floor. Remove the old hardwood and use the softwood subfloor under it. It makes a much better nailing surface. Always use flooring nails every 6-8 inches. Never use floor staples.
A wood plank floor 4″ or wider will only get half the flooring nails of a 2″ board. So to compensate for this you will need to pre bore, screw and peg all the ends down. A few of the longer boards (over 5 feet) will need screws on their mid length. If you are considering installing really wide 6″ and greater boards, it’s best to install longer screws into the joist themselves. You won’t want too many pegs showing so this added screwing should be confined to only the longest boards and should not repeat itself for at least a half a dozen rows. Install the floor by nailing it just like a strip floor. And then go back and mark where off the pegs should go to make sure that the overall effect isn’t too busy.