Another advantage with StumpWM or Ratpoison is that they don't draw window borders and there is no task bar etc, so you have quite a bit more space. And since the WM is controlled by keyboard shortcuts you are actually more efficient.
Unless you really need capitol S Stability, I would recommend running Arch, assuming you're comfortable with a unix environment, or willing to learn. It's a very clean minimalist distro, and there isn't very much to wrap your head around that isn't just common unix stuff.
To put it in perspective, I use Arch on a P3-850-based Sony Vaio with 128mb of Ram and it flies.
The main things you'll get from Arch on a netbook are:
* Rolling release means that you don't have to do distribution upgrades.
* Have it your way - you pick what you want, what window manager you want, what editor you want etc. Use something like AwesomeWM, Xmonad or OpenBox to get the most out of your screen real-estate.
* Efficiency - Because you're only running what you want to run, your memory usage will be better and you wont have things you don't need lying around your hard disk, leaving the space to be filled with things you do.
The downsides:
* It's not a mainstream distro. While it has a great community, it's not as ubiquitous as something like Ubuntu. Still, Arch hasn't yet had it's eternal september so the people you do encounter know what they're talking about.
* You will tinker more and it will take up your time - if you don't play around then everything should work fine, but with the amount of flexibility Arch offers you will inevitably go through Window Managers and various tools, at the risk of breaking something.
Hope this is useful.
This goes beyond the OP's question. How does arch handle changes in essential components like the switch to upstart in ubuntu? Does this work well with rolling releases?
After you get the base installed, just type "pacman -S chromium", and it'll just install the dependencies, and voila.
Then you can just put "exec chromium" in your .xinitrc and type `startx` each time you boot up, if you want to run a browser.
That said, if you're looking for something as quick and easy as possible you might want to look around a bit more to see if there are any prepackaged systems that do exactly what you want with less setup. No sense making this more complicated than it needs to be if something else comes up that closely fits your needs and could potentially save a couple hours of troubleshooting.