Modern Linux systems could do with more than 64Mb of RAM.
So you are artificially limiting yourself in choice, but Mat Alan's suggestion of Puppy Linux is as good as any. However, you need a version prior to 1.0.2. You may get away with it by using a swap partition, but it will result in a sluggish experience, by Linux standards.
Damn small Linux is another alternative, that will run on your computer.