I don't think everyone here grasps what the difference between a Core 2 Duo and Core 2 Quad is. Yeah, naturally, it's a quad core Core 2, but there's more to it than that. If you put "Presler" in google image search you'll see what the Pentium D 900 looks like - two completely independent and separate pieces of silicon, two Pentium 4s, in a multi-chip-module (MCM).
The Core 2 Duo is a single chip, both cores share 4MB of cache, and talk to each other directly. The Core 2 Quad is a pair of such chips, on an MCM, like Presler. So you've got two 4MB caches, with a pair of cores on each. If the two seperate chips want to talk they've got to send stuff to the main motherboard chip (Northbridge) via the FSB. Like two pompous gits sitting next to each other on a bus, really, both thinking they're too good to talk to the other face to face. Or, a grumpy neighbor who only stays in touch by sending a letter by post, rather than just saying "hi" over the fence. There's a good picture
here.
Normally that means the FSB gets clogged, like trying to cram a small elephant down a straw (and then back again), but in real life it just doesn't matter much for a desktop. Overclocking definitely helps though. Servers and workstations, that's something else.
As for AMD chips, they talk using an onboard Crossbar Switch, not unlike a network switch/hub really. The cores, memory controller, and hypertransport link (to the northbridge) are hooked up to this crossbar, and it's all on one bit of silicon. No MCM hot-gluing here. (Technically the cores are connected to a system request queue, which itself is linked to the Crossbar, but that's a bit irrelevant.) It's a pity the cores themselves can't quite match what's in a Core 2 Duo...
AMDs upcoming K10 Barcelona chip still uses that system, with four cores on the system request queue, one HT link on the crossbar (up to four in servers), two memory controllers, and a global 2MB store of L3 cache thrown in somewhere for good measure. Most likely that's on the crossbar as well. If the individual cores in a K10 can match/beat what's in a Core 2, they've got a damn good chip on their hands.
That should set you straight. Oh and by the by, first post!