Tony, do you know of any documentation or other evidence of how Openbox, Xfce, LXDE, IceWM, and fvwm2 stack up against each other as far as "lightness"?
What's your definition of lightness? WM/DE Memory (MB): https://l3net.files.wordpress.com/2014/02/cmp-all4.png What ever versions of those WMs and DEs are compared, it's irrelevant.I experience JWM on Debian and Arch as the environment that fit good to my needs. Most important for me are stability and that nothing annoys me, so for my taste I for example don't want GVFS and pulseaudio. I want to use the mouse, I dislike too many shortcuts.
However, IMO using less RAM usage by the WM/DE is relatively unimportant, it anyway depends to the used applications. Does a lightweight app support the functionality I need? For those who care (too) much about RAM, consider to thin out the kernel too.
Regarding to performance, nowadays, when using modern computers, it's much more important to get rid of shared interrupts. Consider to unbind USB ports that share interrupts with other hardware, consider to mount some cards to another PCI(e) slot, than the one they use now etc., assumed there are shared interrupts.
Linux does handle RAM very smart, to get a faster startup or a faster performance you need to care much more about other issues, than you should care about RAM usage.
JFTR your really oldish PCs likely consumes more power, than a decent energy saving dual-core does, so if you want to save the environment or to save money, a relatively new computer could be appropriated.