The Core 2 Quad and Core 2 Extreme are the most powerful CPUs in Intel's line-up. As the name implies, Core 2 Quad processors all have four physical cores, while the Core 2 Extreme range includes one dual-core processor – the QX6000 and QX9000 ranges.
The Core 2 Quad range includes both 65nm and 45nm parts, based on the Kentfield and Yorkfield cores respectively. Internally, they're effectively a pair of Core 2 Duo processors built into one chip. As AMD likes to point out, this is less efficient than the Phenom's fully integrated design, as cores on different dies can't communicate directly at the chip's full internal speed. Instead, they have to pass data back and forth via the slower front side bus.
But in the real world, multithreaded programming is in its infancy and few applications use a quad-core processor like this. Far more often the cores will be running multiple independent processes in parallel, something the Core 2 Quad is perfectly equipped to do. It's shrewd economics on Intel's part to reuse existing core logic rather than investing on a completely new quad-core process.
If on the other hand your after maximum performance, look to the Core 2 Extreme series. These chips are based on tweaked 'XE' versions of the Conroe, Kentsfield and Yorkfieled cores, running at clock speeds considerably in excess of their standard counterparts. The Yorkfield-based QX9770 goes as high as 3.2 GHz, while the fastest Core 2 Quad with that core runs at 2.83GHz.
It's normal to pay a premium for performance, but the Core 2 Extreme series takes that principle to absurdity.
In turn, the Core 2 Quad series suffers from a similar criticism in relation to the Core 2 Duo. If you can really make use of four cores then go ahead, but for everyday Windows computing the effective performance boost simply isn't enough to justify the price increase over the Core 2 Duo E8000 series.