FC only officially supports New World Macs. Old World Macs are harder to set up; the best distros for them are Debian and Yellow Dog 3. But PowerBook 1400 is also a Nubus computer (i.e. it doesn't have Open Firmware). If Yellow Dog says that it won't work
on this computer, don't even think about using FC, which is geared toward newer computers. You need to figure out how to use the Linux Nubus kernel
; there are some useful links on that web page. Here
's a guide for installing an older version of Debian on Powerbook 1400 that might be helpful. Oh, and that 40mb of RAM might not really cut it for X.