Nothing against DSL, but its a curiousity, its an emergency disk. Its amazing and ingenious, but you'd not install it or use it as your main distro.
Puppy is a possible main use everyday distro with an amazing quantity of stuff in it for the space and very fast on old hardware.
These are both a lot smaller than DIY, Debian + Fluxbox + xfe.. But having thought long and hard about this for someone with quite a bit old hardware, it seems likely that the flexibility you get from the DIY Debian minimalist solution is worth a lot in terms of maintainability, and probably outweighs the extra disk space. I used to like Windowmaker a lot on old hardware, and it is fast, but its stuck in a bit of a timewarp, no faster than fluxbox, no easier to customize and not as attractive. Ion is quite nice if you are setting up an appliance. Its very fast indeed. Again, wouldn't want it every day, but if you're running only one app in a production environment, it might be worth looking at.
Al