No. There are few (But they do exsist) applications written to take advantage of the other Core. I know for a fact that not many games are multi threaded. However Seti can be run in its own instance one per core, F@H too. And they still have to share the Same memory video card and every other resource. However, they are still bery good for multitasking. Its depends what your needs are.
If per say you wil be encodeing Video files or doing alot of DVD's burning / ripping than dual core is a good idea. But since everything is going mutlicore anyways. Theres really no good reason to buy a single core proc anymore.
But to restate. No you dont double the speed.