The difference is noticeable between a dual and quad core. It's true that there's plenty of applications that won't make use of all the cores, but there are a lot that will. Quad core technology has been out for around 3 years or so now (if not more), and they know what they're doing. That means companies have started to use it.
If you do programming, gaming or editing, you'll make use of the cores and you'll see.
AMD and Intel both have out actual quads now (not the old one that Intel used) and they are true quad core GPUs. I have an i7 in this notebook and you definitely notice the difference in it versus even the old quad cores (Q6700) or dual cores.
It really depends on your needs though. If you won't use more than a dual, there's no point in getting the quad.
The basic difference is how many cores are on the CPU. The rest of the differences depend on the model of CPU.
The tasks that you use the computer for, will determine which is better for you. Quad cores have the ability to multitask better. However, if the cores are slow, the dual core may vastly outperform the quad.
Personally, I still use the dual core E8500, and it's still one of the better gaming CPUs. But not everybody uses their computer for gaming.