You don't really need to know xml to use ajax. Definitely start picking up javascript. I've been meaning to write a FAQ in the asp.net forum with a simple ClientCallBack example(the asp.net 2.0 way of doing ajax), so hopefully I'll get around to that next week.
Plus with xml there isn't that much to know. You define your own tags, then you have elements, attributes, inner text, etc. However when you start getting into xslt then you've got some learning to do (which is what I need to start learning).
Javascript is really cool and I think you'll like it when you get into it (I haven't been in it that long).