What you have written alls sounds very plausible (although I do not know much about setting up thin clients so
will leave that to the experts).
My only thing is how you have set your question out. You have some good points that you are querying on but
on reading it all seems to run together and I found myself re-reading parts to get the gist of what you want.
Just a suggestion would be:
This is what I have in mind:
Current hardware: 4 old Hp Laptops, 1.6Ghz CPU, 512 Ram
1. Set up a Terminal Server using Ubuntu 9.10 and FreeNX
2. Run those laptops as thin clients
1. How powerfull should my server be?
As you can see if broken down then people can help on what they now about (ie For question 1 do blah)
As I said, just a suggestion