wood plank subfloor repair and leveling
I would make a cut with a circular saw set to the thickness of the boards all the way around the room, as close as the saw allows to the wall. I'd then fit 3/4" plywood inside the cut, adding framing below where necessary to support the edge well. I'd then overlay 1/2" plywood wall-to-wall, with construction adhesive between it and the old boards. Screw it all down with 2-1/2" construction screws. 

If you're set on leaving the old boards in place, here's what I'd do:


Screw every board down tight with 3" construction screws, two per board per joist. Don't bother replacing any unless they're rotten or otherwise falling apart. 
Grind down any severe bumps or high spots. If the boards are all down tight to the joists I wouldn't expect many.
Run 1/4" beads of construction adhesive perpendicular to the boards and on 6-8" intervals. 
Set your plywood overlay and screw it down on an 8" grid using 2" construction screws, creating a laminated bond with the boards below.