Xubuntu is the lightest Linux distro that's easy enough for any non-IT person to use.
Xubuntu needs 128MB RAM (192 when installing from a Live CD). 300Mhz CPU recommended. You'll need about 256MB RAM if you're impatient though.
Ubuntu recommends 700Mhz and 384MB RAM - But I'd recommend at least 512.
https://help.ubuntu.com/community/Installation/SystemRequirements
Both can use Windows wireless card drivers (with ndiswrapper...just make sure you have some kind of net connection for easy installation).
