Lean and mean? How about Damn Small Linux? Or is that too lean?
How savvy are you on installing stuff? Debian, Gentoo, and Slackware can be as lean and/or as mean as you want them to be, but you have to work for it (everything's from scratch).
If not, 256 MB of RAM is plenty, even with a 400 MHz processor. You could probably go for a nice Gnome-default desktop like Ubuntu--maybe install XFCE as a windows manager (that's lightweight).
In choosing a distro, other things matter more than the programs you use, as you can always install or uninstall programs. What makes a distro different from another distro are these things:
- community support / documentation
- price/availability of CDs/DVDs
- live CD or install CD (or both)
- one CD or many
- lightweight or normal weight (the lightweights are about 50 MB; the "heavyweights" are about 2 GB)
- ease of installation
- hardware support
- default applications
- application installation vehicle
What informs your choice about distro has a lot to do with the type of user you are, what's most important to you, and whether you have dial-up or broadband, among other things. Can you tell us more?