my 2 cents

1. Subfloor should be plywood.  It's a lot stronger than OSB, no matter what your contractor tells you.
2. Subfloor should be fastened to the joist using screws, not nails.  Nails can pop, screws won't.  The difference is it takes longer to drive screws in.  99.9% of the contractors won't do that for you.
3. Before putting down the finished floor, a layer of shock absorber mat would help.  Again, 99.9% of the contractors won't do it for you.   
4. Forget about it.  You are going to upgrade in < 7 years anyway.  Put your money into cosmetic things, like Tiffany light, window treatment etc.  You get much better return on those investments.   
Just my 2 cents.