If you are going to paint the crown molding take a look at this product. I used it in a few rooms in my home and have just ordered more to finish my bedrooms... It will work great on uneven ceilings, does not need nails, looks great when painted and does not need fancy cuts or much work to install. It was also used on the most recent This Old House project....
www.trimroc.com They also have an installation video online so you can see how easy it is to install! Good luck!