Help! How much DNA to use in IP reaction? - (Mar/24/2011 )

Hi all. I am just getting started with ChIP experiments and have a question.

I've read a bunch of different protocols, but I'm still a little confused about what is a good starting point for how much DNA to use per IP? And also, should I go by DNA concentration or cell number? Cell number seems easier since you don't have to use a bunch of your sonicated sample to determine DNA concentration, but do you base the cell number off of what you collected from culture?
I just want to make sure I know what I'm doing before I get started.
Thanks for any help!
-D

I use ~2x10^6 cell equivalents per ChIP. I know you can go lower though....