You didn't give enough information to properly identify what it is you have.

This is the acer extensa 4220-2555 with celeron M 540 @ 1.86GHZ

Acer Extensa 4220-2555 - Celeron M 540 1.86 GHz - 14.1" TFT specs and Laptop specifications - CNET Reviews

The only thing upgradeable is the memory to 2 gigs.

I expect the same thing for the dual core model as well!

Google is your friend....post the name. series and model in the google bar and see what it comes up with!