I'm new to hardwood floors but I am a fairly experienced DIY'er. I'm planning to install 5" hickory pre-finished wood (3/4" thick) in my living room and den. Each room is ~ 250 ft2 while one is over a basement and the other is over a crawlspace. Both currently have carpet over 5/8" particle board underlayment and 1/2" non-tongue and groove plywood subfloor. My questions are:
1. The particle board is not suitable to nail into correct?
2. I can replace the particle board with 5/8" plywood correct?
3. Considering that I'm in Ohio (not to wet or dry), how long should the plywood be installed to allow for moisture content to equilibrate?
4. For the room over the crawlspace, what should be done in terms of vapor barrier? Is putting plastic over the surface of the ground ok? Should it cover the cinder block walls in the crawlspace? What should be done to the floor before installing the hardwood, roofing felt or something better?

I want to do this right but also don't want to waste time and money on overkill. Thanks for any help that you may be able to give me.

1. No
2. Yes
3. Acclimation is not a time thing. It might take a few days, it might take a few weeks....invest in a good moisture meter. Check the moisture content of the existing plywood and shoot for that.
4. The crawlspace is the more problematic area. The ground must be covered completely with a vapor barrier (6-mil poly, etc.) and have adequate cross ventilation. So, get the crawlspace in shape first and foremost. Then you'll need to let the existing subfloor equalize in that room before continuing on. I would check the MC in the subfloor of the room over the basement, and wait until the existing plywood in the other room equalizes to approximately the same mc. Then install your new layer of plywood on top, and allow it to acclimate.

Once the subfloor has been addressed, you'll want to get the hickory in the house, open it up and let it acclimate. Hickory is one of your lesser stable species, so you'll want to be completely sure it is acclimated properly before you install it. On 5" width, it needs to be within 2% of the subfloor. So, if the subfloor MC is 8%, the Hickory should be no less than 6% and no more than 10%.

Follow the manufacturer's recommendations, but typically you'll want to use 15# asphalt saturated felt paper over the sub before installing.