I've heard generally positive comments about all three brands you mentioned: IBM, Dell, and Sony. Though I've never used a ThinkPad, they should be constructed well and be reliable.
I've found Sony's laptops to be quite expensive, but otherwise they seem to be pretty nice. If you can afford it, go for it!
Dell doesn't have very many thin and light laptops in its Inspiron series, but the Latitude X1 breaks ground in the ultraportable category with an extremely thin profile, excellent keyboard and good performance.
If you're going to run one program at a time (e.g. Internet browsing or word processing seperately) and have minimal background applications running, 256MB of RAM should be enough. Upgrading to 512MB will result in a noticeable performance boost, and beyond that really isn't necessary based on what you're going to use your laptop for.
You're not going to require dedicated graphics memory; while it'll be nice, they'll consume more power and thus degrade battery life.
I'm looking for a thin, reliable laptop and I really just use the basic functions like IE, all the Office stuff. I definitely would like one with fast processor speed, more memory and of course wireless capability.
I'm deciding between IBM Thinkpad T series, Dell and Sony Vaio - looking for some feedback if you're users of any of these.