The R520 will have SM 3.0.
It is only necessary if you want the advantages that come with it which is more eye candy and better performance.
If you want "more" future proof when you upgrade assuming you are not going to buy the R520 then it is better to go Nvidia. If you don't care about the advantages of SM 3.0, ATI GPUs in general still beat Nvidia in Direct 3D games.