It is ALWAYS better to remove the old paper first,but if impossible skim out the seams and prime with Zinsser"Shield's and hang over it

Here is a link that might be useful: stripping

When you eventually need to strip two layers, with the bottom one painted you will really hate the job.

I'd suggest trying a steamer to remove the wallpaper which was not mentioned in the link provided by christophern. We've found that to be the best method. It is a pain in the butt no matter how you do the job but it is way better to remove the old paper rather than doubling your future troubles.

Wagner makes a wallpaper steamer and sells it for about $40.