What is actually the difference between deactivating 2 of the cores as in this screenshot and having a dual core machine
I guess is not the same as having a dual core machine, right? Do you know where I can find some information related?
You're only telling SQL Server 2008 itself to only use 2 of the available cores. The rest of your system will use all 4 cores.
If you had a dual core CPU then your whole system would only use just 2.