It all depends on how long you've been on the Xanax. I advise you to do this under the care and supervision of your doctor. Withdrawing from powerful benzos is not something you want to try on your own. Xanax is probably the most difficult benzo to get off and keep in mind that not all drugs are equivilent in potency just because they both say 2mg.
Valium does not really have much affinity for panic if that is what you were taking the Xanax for. It is used more for mild forms of anxiety and I don't recommend it as a cross-over med. (And I don't care WHAT Elizabeth Ashton says!)
If you to take a benzo long term, discuss with your doctor getting on something like Klonopin. Both Ativan and Xanax are for short term use ONLY.
I think that no matter what transition you choose, you should be prepared for some discomfort. ALmost nobody comes away from this kind of transition unscathed. But a good doctor can make it far more comfortable.
I wish you luck
Peace
Greenlydia