noise - How do I choose a ferite bead for power supply filtering?

I want to put a ferite bead on positive SMPS supply rail that goes to analog part of the mixed digital/analog circuit.

Now on parametric search at Digikey I can see several parameters: Current rating, DC resistance, Impedance @ frequency. I understand current rating and dc resistance, I understand impedance, but how do I make my choice - should I look for the greatest impedance @ 100MHz with acceptable DC resistance and current ratings for my application?