I recently upgraded a 12" iBook with 256mb of RAM and a 30 gig hard drive (I believe), from Panther to Tiger. My friend reports that the computer has never ran faster.
I surely have high hopes for Leopard, it's make or break for Apple, especially with their market share on the upswing.
I purchased a 12" powerbook 1ghz and I love it so much I may not buy a new MBP for a while. I would like to put leopard on it when it comes out and I was hoping it'll be able to run it for a while...but I still want an MBP....such a dilema...