I am tearing out the old oak flooring out of the living room and going to install new through my new kitchen and living room.
The first problem is the uneven floor. over 20' it dips down 1/2" at the 5' mark back up in the middle and down again and back up again at the edge.So basically at the walls and middle it is even and has two dips in between. Seemed to be o.k. with the old floor and I'm told that it shouldn't be a problem. Iv'e been here 6 yrs and never noticed it other then when your trying to level a free standing shelf against the wall. The old floor joists are in good shape, they seem to be just different thicknesses.

Will this be a problem.

Should not be a problem.

Well, it may not be a "problem" but since you are going to a great deal of effort to put in a new floor, why not take the extra steps and flatten the subfloor out prior to the new installation. From your description, it sounds like you have two depressed areas in the floor which can easily be packed up with cork, cardboard, levelling compound, wooden shims; whatever. Alternatively, if the joist system is exposed underneath, using shims on the lower joists will solve the problem as well.