1) Subfloor is made up of a layer of 1/2 inch plywood (attached to joists) and then 5/8 inch particle board on top of that -- Is this a sufficient subfloor for 3/4 inch hardwood flooring ?? If not, what do you recommend doing ??

2) Want to use 3/4 hardwood for large room -- The floor joist run one way for half the room and the opposite way for the other half -- Would like to run hardwood all the same way -- Is this a problem ?? If yes, any recommendations to remedy situation ??

It is not recommended by most professionals and assoications to nail through particle board as it does not have very good holding power on fasteners, but as far as support and minimum deflection I dont think its a problem.

Remove the particle board!!!!!!!!!!!

Another overlay of ½" or greater plywood.

Take up the particle board and replace it with 5/8 square edge plywood. That will give you a 1 1/8" thickness, and allow you to run the floor any direction you want if the joist are spaced 16" OC.