I think a while back I needed to find the magnitude of a
          vector.  I found that if I scale X and Y to a range from 0
          to 1, I can find a quick answer (I think worst case was 12%)
          if I do this

          max(X,Y) + .5 * min(X,Y)

          for sqrt(X^2 + Y^2).

          Once again, it is imperative that you scale the values to 0
          to 1.  I think multiplying he min by 1/3 gives better
          results, but multiplying by 1/2 is so much easier.

          Of course this is all off the top of my head and is only
          valid if memory serves me right.

          -Edwin