This just is not the case. The entire Nehalem architecture is improved from the integrated memory controller (good riddance, front side bus) to hyperthreading on the i7, turbo mode and the ability accomplish the same tasks as a Core 2 Duo with the same amount of energy or less at higher speeds.
This translates into performance increases in everyday apps. The performance boosts in apps coded to use quad cores goes without saying.
"Notice" is an entirely subjective debate. Just from personal experience, as I mentioned, I upgraded from a Core 2 Duo 2.4GHz iMac to the i7 and it is the most noticeable boost in speed between computers I can ever recall in 20 years of using them. I'm not referring to video editing/encoding apps which I don't even use or benchmarks. I mean from the startup of the machine to individual apps like iTunes and iPhoto.
It smokes! Not literally, thankfully.
If your budget is a Core 2 Duo machine and that does what you need it do then that is absolutely the machine you should get.
On the other hand, if you can afford a quad core and plan on using the machine for the longest period of time possible I would absolutely recommend it.
But to say that the extra $300 is a waste of money which you won't notice any performance increases in unless you're a video editor is just not true.