I am wondering if it is easy to do this. I know that using X to serve the other computers ain't that difficult. I just have no idea how would I have to configure the laptops in order for them to work as terminals.
If your older laptops can boot from their network cards (ie. PXE boot) then you can set up LTSP, which is fully supported in Ubuntu:
Here is a pretty old post which I wrote: http://ubuntuforums.org/showthread.php?t=1437074
You will have to copy commands line by line from the script cause from 10.04 it doesn't work any more automatically by dropping into the Terminal this script.