Go for Vector Linux.
It is based on slackware.
It comes with IceWM, FluxBox and XFCE4.
Minimum config suggested for it is pentium 32 mb RAM and 950mb of disk space.
It has 3 editions. It is well optimised for old hardware.
It has some good system management tools like VASM, Gslapt.
But, you cannot run GSlapt without root permission and there's no indication of that. Same applies to USB drives.
You can get more from
www.vectorlinux.com
Here are the system requirements for it *distro.ibiblio.org/pub/linux/distributions/vectorlinux/docs/miscellaneous/version_table_2.html
Vector Linux 5.1 standard will do pretty good for your configuration.