What was the last CPU upgrade that you really noticed? (-"upgrade" to include getting a new computer.) I recall how nice a 586 was compared to a 286. The step up to an Athlon was impressive. After that, the improvements weren't so dramatic. I'm configuring some Phenom 4 core machines and they don't seem any faster than my AMD 3 core machines. (It's an FC 16 vs FC 12 comparison also.)
Maybe there's a corollary to Moore's law. Transistor density will double every two years, but you'll notice it half as much.
Of course, I don't do computer gaming or anything computationally intensive. Perhaps people that do still notice dramatic improvements with a new CPU or machine.