I've sort of adopted boxing terminology for hardware requirements.
As I mention in another thread, I've really gotten to like Vector as a "welterweight" Linux, something I will try on a machine with 64MB or so of RAM. Especially if I'm expecting non-Linux-geeks to be able to use it easily.
Most modern newb-friendly linuxes, in my experience, can be installed on machines with 128MB of RAM, "middleweights" (except for IBM Netvista 6841's, even with 1GHz P-III cpu's, they behave more lie a welterweight machine) More RAM is of course always better.
Lightweight: D*mn Small, Puppy. Of the two, I like Puppy better.
Welterweight: Morphix, Wolvix, FeatherLinux, Mint XCFE beta, Vector, most Knoppixes. Often advertised as "lightweight GUI" 64+MB
Middleweight: Fedora 7, MEPIS, Mint, Ubuntu, Kubuntu, PCLinuxOS 128+MB
Lt. Heavyweight; SUsE. 256+MB
Heavyweight: experimental 3D interfaces. Or WinXP
My current installs:
Personal desktop: 2.4GHz Celeron, 640MB ram. Still WinXP. I paid for it, I'm going to get maximum possible use out of it, d*mnit!
Laptop: Compaq Presario V2000. Dual-boot, WinXP/Fedora 7
Basement desktop: 500MHz P-III. 256MB RAM. was Fedora 7, now MEPIS (only because MEPIS has berkeley's BOINC client packaged and available in their default repo.)
Coffeehouse internet terminal: 450MHz P-II, 192MB RAM. was Vector, now PCLinuxOS. If the shop owner lets me, I'll convert his stupid Win2k NetVistas to Vector.
I tend to prefer Gnome-based to KDE-based, only because I don't like how Konqueror adopted MS Internet/Windows Explorer's tactic of having hard drive and web browsing being handled by the same app. On the other hand, K3b kicks Brasero's butt in the CD burning department...so it kind of evens out. I've flirted some with XCFE. When it mimics Gnome, it's ok, but some XCFE-based desktops (like Morphix) are a little too avante-garde for my taste.