Newbee and Has2be have provided excellent advice, and my hat is off to them.
The only additional thing I would recommend is a 1/4" layer of luan on the plywood subfloor, which is how I went.
As far as underlayment goes, rose paper is the material of choice these days. It also makes the job go easier, as the hardwood glides across it easily.
I'm particularly happy with my own installation in my home, which followed this route, though I definitely noticed the rooms are noticeably colder in the winter time in comparison to when I had wall to wall carpet. And, because I was so happy with the previous installation a couple of years ago, I'm now about to do one additional room in the next month, and am considering replacing the rose paper with either asphalt roofing paper, or another product called "Insulayment" to address the winter time feel issue.
And, by the way, if you lay down an unfinished floor, I more than recommend finishing it (yourself, not a contractor) with polymerized tung oil from Sutherland Welles, who Rushton recommended to me. Once you see a floor finished with this material, polyurethane will NEVER look right to you again. While the per gallon cost is probably twice as much, it went so far, I only needed about 1/4 to 1/3 of the estimated amount, so the actual cost is much lower. And, with tung oil, you can refinish wherever/whenever you want. With polyurethane, you cannot make spot repairs, the entire floor must be sanded and then refinished.
Best of luck!