Do I file an I864 for wife to join me?

[Kermit07]

I am on H1B presently. My wife will join me for the validity period of my approved I129 petition for non immigrant worker valid thru May 1, 2007. I have two questions and both pertain to an I864 affidavit of support.

Must I file an I864 now for her to join me?

Must I file an I864 later if and when we apply for permanent residency?

A little confused on this issue. Thanks for helping. Hope to see some responses very soon.