I've used both 2 and 4GB Chromebooks. As to disk space I haven't needed any since all the docs and such are "in the cloud."
Here's my tip. Be sure to look on Groupon for Chromebook deals as these are returned by many and end up being sold cheap. If you just need a good laptop, my brother and I picked up a few like these for the office. Two links follow. one for a chromebook and the next for an everyday use laptop for web and more.
Given they cost about the same, I'd get a normal laptop.
Thinking about replacing my older laptop with a Chromebook. I'm used to having a fairly large HDD. How much memory do I need?