Linux-blog.org has tested SimplyMEPIS 3.3.1, Ubuntu 5.10 and a bunch of other desktop distro's to see how they fared with XFCE on it.
Read the interesting investigation
If I had to recommend a desktop distro for a slower PC like this one, I'd recommend both Ubuntu AND VectorLinux with Vector being the first recommendation. Vector is just quicker than Ubuntu on this platform and supports many different media formats out of the box. Sure, Ubuntu provides nice how-tos and tutorials to get things up and running, but Vector has VASM and that provides so much control over your environment…not to mention that Vector supports media on the fly. It just seems logical that you'd go with the faster one that you have to do less to in order to get a working media rich desktop.