Check out US Department of Energy recommendations by zip codehere. (Click first link.)

Dr. Lstiburek of Building Science recommends min R60.

If you have the space, I'd suggest doing the work in this order:

1) Seal all air gaps between the attic and interior. In the '50s, there was virtually no attention paid to this.

2) If there is a coat of oil paint on the ceiling, you may forego a vapour barrier in the attic.

3) Provide sufficient roof and soffit vents.

4) Provide that the air route to the soffits remains clear. Use, for instance, pre-made styrofoam baffles.

5) Insulate with blown-in cellulose to a minimum of R 40.