Dear Dave
Most assuredly you must remove the particle board. It is the poorest of nailing surfaces.
Add another layer of 1/2" plywood, and this will be plenty. Be sure to fasten this layer with 3" spiral nails nailed into the joists. Apply squiggles of a urethane construction adhesive on the old plywood at the joist and between the joists. This will prevent the layers of plywood from squeaking at the nail shafts. A thinner plywood will not provide enough structure.
Be sure you use proper flooring NAILS, not staples for the new hardwood. Read my article about this in the Floored News section (Primatech Article).
As always your Most humble servant, Joseph, the Wood Floor Doctor.
You may also find these articles helpful:
1. How To Repair A Hardwood Floor For D.I.Y
2. Should I sand the wood floors myself or hire a pro
3. How to remove various stains from wood floors
4. Stopping Those Annoying Squeaks In Your Wood Floors
5. How To Take Care Of Your Health And Safety when Installing, Finishing, Repairing or Cleaning your wood floors
