I actually tried installing Xubuntu on a really old comp with 95 MB of RAM. When I posted a question on the Ubuntu forums about an error I got, they had a few suggestions.
You can go
here for a way to install it on a low memory system.
Also, there is Ubuntu Lite and D.amn* Small Linux.
*There isn't really a period between the D and A, but I had to put it there because of the language filters...