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.