Best Lossy Sound?

Hey guys, I'm experimenting with Itunes on getting the best sound out of my higher end headphones. I've noticed 128K sounds terrible, and I'm not getting much better at 192K. Right now I'm using 256K with Variable Bit Rate encoding. Will I be able to tell much difference between this and a cd. If not, what is the cut-off point for not being able to tell the difference?