Since you're probably accumstomed to regular notebooks, I'd recommend a convertible form-factor. And if you want to carry it around and use it like a book (around the house, standing around the D&D table as DM), I'd recommend getting the lightest one that meets your needs.
My Fujitsu Lifebook T4010D is pretty light at 4.3 lbs if you swap out the modular optical drive with a weight saver insert. I wish it was even lighter.
IBM just came out with a thinkpad tablet weighing in at approximately 3.5 lbs, supposedly the lightest convertible available. Unfortunately it doesn't include an optical drive.
Most tablets only support 1024x768 which is disappointing, but I've gotten used to it.
