You will see a performance boost, but just don't expect to go from a Volkswagen to a Porsche.
The Duo Core processors, as well as the AMD AM2 processors are built for multi-tasking abilities. Don't get me wrong, you'll definitely get a "speed boost" by upgrading from a single core processor to a dual core, but where these processors really shine is multi tasking abilities. You'll be able to have multiple programs open and operating at the same time without a degredation of processing lag and a significant boost in gaming, but I think that most people think that they're going to get double the performance just because they are dual cores.