From my experience using a 603e and a G3 in a 1400 the difference is quite obvious. I was lucky enough to pickup a 1400c with a Vimage 233mhz G3 upgrade complete with 64MB RAM (for $25 with a spare logicboard). I have since upgraded the 2GB HD to 10GB and installed 8.6 and it flies. I have a 1400cs with 8.1, stock 133mhz 603e and 48MB RAM and it is nothing like the G3 233mhz.

Now the price may not really be worth it in most cases, but if you are lucky enough to find one for the right price I would definitely pick it up