Well, first of all, the upcoming Gulftown is a 6-core CPU with Intel's HyperThreading.
HyperThreading allows each core to execute two threads simultaneously.
Your computer would see this CPU as having 12 cores - in reality, its 6 cores handling 12 threads.
From wikipedia: A thread is contained inside a process. Multiple threads can exist within the same process and share resources such as memory, while different processes do not share these resources.
I'm no programmer so that's as far as my knowledge extends. I think of them as "sub-processes" as they exist within the process itself (explorer.exe, firefox.exe). Applications that are "heavily threaded", as in they've been written to take advantage of multiple CPU cores, benefit greatly from multiple cores and even more so from features like HyperThreading.
Core 2 Quad's are strictly 4 CPU cores - no HyperThreading.
They can run 4 threads concurrently.
I'm sure an experienced programmer can explain this better. If you're running something like Photoshop or Maya/3D Studio Max, the more cores the better.
Games are beginning to take more advantage of multiple cores (see: GTA4) but generally, beyond dual core, performance isn't hugely improved by going with a quad-core.
2 cores with hyper threading appear as 4 logical cores in your system, but they are not the same as a true quad core which has 4 physical cores. hyper threading allows a core to do 2 task at the same time as they have 2 threades on a single core, but those 2 threads share the core memory, it is faster than a dual core w/o hyper threading of course specially in applications or softwares that can run multiple threads, but its not as fast as a true quad core that has 4 physical cores, because those threads dont share memory, they have their own core memory, so they can do task independently. its like comparing to a person, a dual core w/o HT are 2 person each carry a sack of rice , while dual core w/HT are 2 person that each carry one and a half sack of rice for a total of 3 sacks of rice. while a true quad core with 4 physical cores are 4 persons.. so you know who wins.