I was using a 633MHz PIII until about 3 y ago, then using it again recently when my main computer died, and its gotten a lot harder (on Linux) over those 3 y.
No, ubuntu specifically on that machine. The only tricky part as I can see it should be the webbrowser. You'd either have to use an old one, or a featureless one.
I was using an old one (Firefox 1.5) and it was extremely painful with Amazon.com. Amazon.com seems to have changed in the 3 years since I used it last with that browser.
I always thought that a "featureless" one (one without modern JS or CSS) would be extremely painful or useless, too.
It's amazing what you can do with some swap and a little patience. ;)