Linux, who art thou?

Lately I have swamped myself with thoughts on which Linux distributions I want to try. I want to try Ubuntu and Kubuntu on my home-built machine, as it is the most powerful and graphically capable. I have used previous versions of the two, liking Kubuntu more for the K development environment. I also have tinkered with Knoppix and openSuse, which seemed to be somewhat bloated with useless features. But what about the spare computers... the old junked ones sitting in the corner with out-of-date hardware? Well now I have to find a lightweight distribution that has everything I need and runs well, even on an old machine. The list that I've come up with includes distributions such as Puppy Linux, DSL (Damn Small Linux), Xubuntu, Mint, and gOS (Good OS). But which one do I choose? I don't have a whole lot of experience with Linux, and installing and playing with all of them might be more of a hastle than I really want to go through, especially when dealing with old haggard hardware.