check out Xubuntu.

http://www.xubuntu.org/

From their site:
You need 192 MB RAM to run the Live CD or 128 MB RAM to install. The
Alternate Install CD only requires you to have 64 MB RAM at install
time. To install Xubuntu, you need 1.5 GB of free space on your hard
disk. Once installed, Xubuntu can run with starting from 192 (or even
just 128) MB RAM, but it is strongly recommended to have at least 256 MB
RAM.