With laptops of this era you only have two sodimm slots and DDR2 4GB RAM modules were quite expensive when they were new. So even still today they are more expensive than you would think. I sold mine for $25/each a few weeks ago.

Also some laptops of this era didn't support more than 4-6GB of RAM in the firmware. I know there are several models of early intel macbooks that you can physically install 8GB of RAM but they will not recognize it.

On the other hand, I have a 2010 iMac with all 4 DDR3 sodimm slots full giving it 32GB of RAM. It was a "just for fun" project before the AI prices. Those era iMacs are fully upgradeable (CPU, RAM, and GPU). Swapped in an i7 CPU, AMD m4000 GPU, and an SSD. Runs linux mint great.