I've been working on this problem for ages, here are some thoughts. My problem is I love having little notebooks and a couple of coloured pens, for sketching, prototyping, making notes there's nothing quicker, but it's paper goddammit, and there's no undo.
For actual coding I use either a PC with a couple of 1080p monitors. I could get higher res or larger ones, but for the price would of shot up. I also use a Sony VPCZ1 laptop. It's 13" but also 1080p. It's actually a little too hirez for general stuff, but it works perfectly for coding.
Right, here's the interesting bits.
To replace the notepads I've tried the following.
LiveScribe pen. This is a pen with an onboard computer. It watches what you write and records it as an image. It uses special paper, but you can print your own pads of the stuff so you're not tied to their stationary. It works pretty well, but it becomes a problem when brainstorming with people. If they don't use your pen you don't get their notes recorded...
iPad. I found using your finger just doesn't feel intuitive or accurate enough, so now I have an iPad stylus. It's about the size of a pen and helps. This is currently my preferred route for digital sketching.
Wacom tablet. Not portable, well it is, but it's not the sort of thing you want to carry or pull out in a meeting, on the train, etc.
laptop with touch screen. I've got an HP with touchscreen which you can swivel round to be tablet format. I actually don't like it, I can't put my finger on why, it feels a bit laggy perhaps, strokes with the stylus aren't recorded in sufficient resolution I think. The applications I used with it weren't optimised for stylus use either, unlike the ipad apps, so it felt more clunky than it should be.
Paper and pen + scanner and doc management software. I own a copy of PaperPort which I use for all my document management stuff, so it wasn't a killer step to scan paper pages and put them in there. Still a bit of a pain. I use the red and black series of hardback notebooks in a variety of sizes, everything from an a6 I keep in my jacket pocket up to A4. Oh I use the four colour bic biro pens. I like to use different colours :)
Oh one extra thing, recently I've started using an iphone app called jotnot or something like that. There are loads of them, all it does it process a photo for use as notes. I use that to photo notebook pages and then email them to myself.