Without knowing the board, it is very difficult to tell you which CPU you can think of buying?
First to check is:
Board CPU Compatibility website.
Then check the CPU details on the website:
All the best,
There are four types of dual cores as far as i am aware of!
here is the page to tell you about three of them: http://www.techjunkiez.com/2008/03/10/difference-between-intel-core-2-duo-vs-intel-dual-core-vs-intel-pentium-d/