It depends on the age of the laptop.. when you say "old", how old?
The motherboard has a certain socket configuration for the CPU, so only CPUs with that socket will fit the motherboard.
CPUs are NOT soldered to the motherboard. This isn't the 80's. Did you really mention the 8088 CPU kschang? Really?
You don't have to change the motherboard; in fact you CANT! No other motherboard will fit your laptop other than the one that came with it or one from an upgraded model of that same laptop in which case the only upgrade would be CPU, RAM and HD size with the exact same motherboard.
You don't have the change the RAM. What would the RAM have to do with changing the CPU? Nothing.
I bought an Acer for $300 at Best Buy, it came with a lame single core CPU. I bought a faster dual core CPU on eBay for $20 and installed it in about 30 minutes. Here's how to do it on my laptop, step by step:
So tired of people who say "just spend $800 on a new one, you can't do anything yourself".