But the HD DVD and BD cases are slimmer than the regular DVDs. Just a thad thicker than those very slim ones (but I like the occasional booklet, so I think this is fine).
Mike, The width of two slim cases is about the same for one regular dvd case. While the width of two slim cases is slightly more for one HD DVD or Blu-ray case. Also, the HD DVD/Blu-ray cases are about 1" shorter in height than both dvd and slim cases.
Why not get rid of your girlfriend, and then you can add another bookshelf...just kidding! Have you tried the Case Logic cases for DVDs? They can work wonders, I have over $2,000 DVDs, and the Case Logic cases saved a lot of space for me.
I remember when the Superbit version of "Panic Room" was released on a very thin cardboard case, I thought it was the coolest case yet, but other's complained about it.