I've never tried it before, but if I was looking for a lightweight distro, I'd probably start with Puppy Linux since it has a good reputation as being one of the lightest distros.
The great thing about Linux is the distros are free so try a few and decide what you like. As a minimum, I'd suggest trying an LXDE desktop distro like Lubuntu and an Enlightenment distro like Bodhi. I actually played around with Bodhi a few weeks ago and it looked really slick and interesting. However, I prefer a more feature-rich desktop so it wasn't for me. But I could see how a lot of people would be really happy with the newest version of Enlightenment that is in Bodhi.