The easiest is to have one primary partition for the root partition / and a swap partition. Note: the norm with the swap partition was 2x ram size, but with todays pc 512mb is more than enough.
As for the OS, don't just look at a OS for your mother get one you'll be comfortable with. (You'll be admining the pc) your choice is good or any OS using KDE for the WM window manager.
I put together a pc for my mother using Debian /Etch all she uses it for is going online so all she really needs to know is how to open a web-browser. She also like to get the weather the first thing in the morning, so I added a weather monitor to the task bar. As I have time I plan on setting it up so I can vnc into her pc so as not to have to drive 60 miles to work on it.
we must have replied at the same time, you beat me by this >||< much