How fast is the processor and how much RAM do you have? I'm running Slackware 9.1 on a AMD K6-2 400mhz with 256 MB RAM. (oh yeah.. it's in my sig, I forgot)
What I figured is that 400mhz/256MB is good enough if you run one or two apps OR one app with one or two windows at a time. If you do what I call "light multitasking" (few browsers windows, e-mail client, another app of your choice) you'll need ~1ghz/512MB. If you're like me, and you have lots of windows open, use lots of tabs, and like to leave everything up and running all the time, you might want >1ghz and >512MB.
Damn Small Linux is going to come with fluxbox. Nice and lean (at the beginning)... but not as easy as GNOME or KDE. From my experience, I find almost all window managers/DEs to perform at about the same speed.
why? I run Firefox, and Konqueror, and KMail, and gaim... etc etc. GTK/GNOME, KDE/Qt libraries get loaded up anyway.
I like KDE so I use KDE. I'd probably recommend GNOME for new users since their philosophy is to keep things simple. I don't know how fast the latest GNOME is. Also, with Nautilus now being spatial and all, it might
be easier for new users (everything is an object, don't worry about file heirarchy, etc) Personally, spatial doesn't do anything for me. I don't use the file browser much, but if I do, I find myself moving and resizing windows around anyway. (It's good for simple repetitive tasks though).
My advice is to watch your mom use her computer doing her regular tasks ("photo editing/online bill paying/e-mailing/music stuff.") You can set up Windows XP to act pretty spatial. Open new folders in windows. Remember folder positions, etc. See if she likes that better than regular "open folders in the same window" file navigation.
Music stuff.. what do you mean? she plays mp3s on the computer? listens to online radio? etc? And photo editing... she has a digital camera that she plugs into her computer? what kind of photo editing software does she use? has she tried GIMP?