I do most of my prep on the computer, except for mapping (after trying a dozen programs and approaches, I found I just prefer to do maps by hand). I use Google Docs for all my notes and such, so I can access it from anywhere to jot down ideas when I have them.
Before sessions, I'll write out all the main stats, NPCs, story beats and such on a couple of sheets of paper for use during play. I tried using a laptop for a while, but I just can't get it to jive with my DMing. I still have a laptop open to the side, so I can dig into it for more detailed notes when called for. I often won't, though, and the computer will just sit there for the entire session unused... humming and taking up precious space. I still prefer having it; those detailed notes are too precious when needed.
BUT: One of my players bought an iPad a couple of months ago and she's been letting me use it during play. And I find it quite lovely. While I'm still just accessing exactly the same Google Docs as on the laptop, it just feels more natural. For the last few sessions now, the only things I've had on paper were the maps. We've set our sights on getting one ourselves as soon as the budget allows.