as you know there are products for some computers to upgrade them a generation or more but for a consumer level computer like an ibook, it would make more sense to buy a new ibook because the cost of such an upgrade would not be worth it
the best motherboard to have is one designed for that specific processor to get the best results
i used to have a powertowerpro 180 model which had a 185 mhz PPC 604e chip in it and when the G3 tower came out, this machine lost most of its value...when the G4 tower came out, then the machine was worth three hundred bucks
for more than that, i could have bought a G4 card for the tower and upped the speed of the machine but i had to deal with a motherboard with a slow bus which was not condusive to having a G4 chip in it...to make it work better with the G4 chip to allviate the worst bottlenecks, i would have had to buy more ram...all in all, i could have bought a used apple G3 power mac for cheaper and had an overall better and faster machine
upping an old mac with a new processor card rarely pays off since you can buy a next generation computer used or sometimes new cheaper than a processor upgrade which would often require other upgrades to make everything work at its best...some also buy the latest and fastest video card as well as more ram and a later operating system which a new processor often requires for optimal results
that being said, some people do not care about getting the best deal and like to modify stuff just for the sake of modifiying and many find this a fun way to pass the time...although expensive and unproductive most of the time
...other people like to stretch the life of a computer just for kicks and blow people away with using an antique...i have a may-1999 pc laptop running windows 98 (first edition) and the thing is maxed out with 160 mb of 66 mhz sdram and the machine is equipped with a 2 mb video card...of course not all software products i put on this machine work fully, but i find it a curiosity to see what does work...i downloaded ie 5 or 6 and outlook express 6 on it and the thing works beautifully...when i bring this very thick and extremely heavy monster (8+ lb with adapter) into programming classes, people are amazed that i can get compilers on this thing to work or even fit...the hard drive is 4.3 gb and i have to delete programs and apps from it to make way to fit other stuff...i don't know of anybody who has such an old pc laptop in working condition and i will try and keep this thing going to the five year point and beyond
i know of a man who uses a late 1997 or early 1998 era pc laptop with windows 95 and only a floppy drive on it and this thing is two inches thick and has a 10 or 11 inch passive lcd screen on it and maybe i will go down to his store today and see if he is still using this beast...it's a toshiba and consistently, that's the only brand of pc laptops which last such a long, long time because they are built like tanks but lack some of the desgn coolness and features of a sony