Which GNU/Linux distribution for an old system from 1999

I have an old notebook which has similar configuration:

-512 MB RAM
-40GB HDD
-Pentium M 1,5GHz
-Intel integrated graphics

I recently installed XUbuntu 12.04 on it.

XUbuntu itself is pretty light. After boot it uses around 250MB and overall performance is decent/good.

However when you start using Firefox (I believe other modern browsers will be similar) you will have a big performance penalty, mostly due to lack of RAM. Depending on amount of tabs and the pages you are browsing, you'll have swapping.

Using Debian/Ubuntu alt you'll have a lot more control over what is installed, you can choose an even lighter desktop/graphic environment. On the other hand, you'll probably have to configure more stuff than with regular Ubuntu. But if you plan to use a browser to view anything but simple HTML pages low on images, sooner or later you'll run out of physical memory. It is, in my experience, what eats up more memory and what degrades performance the most.