I'll just observe that the difference in clock rate between a 3.6Ghz and 4Ghz chip is 400Mhz. The
maximum that the quad is therefore slower
in a single thread/theaded app/game is therefore 400/4000 = 10%. (In reality, the difference will likely be less than even that, because overclocking the CPU does not neccesarily mean that the rest of the system is similarly overclocked/overclockable, so for example for RAM intensive apps the CPU might end up just "waiting faster" for data to arrive from the RAM.) So, the point is, even for the single threaded overclocked case, the quad is going to be
at most 10% slower than the dual core. For a single threaded 100FPS game on the dual, that will be 90FPS on the quad. For a 50FPS game on the dual, that will be 45FPS on the quad. Etc etc. It seems to me that it is unlikely to be noticeable, as others commented already. And then this does not even take into account the fact that with a Quad your cpu intensive single core apps will have 3 cores taking care of the other 40+ (mostly idle, but still) processes on the system, which means that it will probably get slightly closer to 100% of one core's time than it would even on the dual core.
As for power consumption, the Duo's are specced at 65W, while the Quad's are specced at 95W (the newer ones anyway.) So the difference is a measly 30W, hardly one lightbulb, and in real terms neither value is exactly huge. (Think back to the days of the hot P4 Prescott's and how much power *they* used...) You could have course turn on all the power daving features (e.g. SpeedStep) which would further reduce the power consumption figures. However, power usage does not increase/decrease linearly with clock rate, so it would not be unreasonable to expect the power usage to be substantially less than half the normal rating at half the clock rate, for example say 20W and 30W (leaving a difference of only 10W) when run at half the clock rate, hardly something to get hugely wound up about.
Finally as others have pointed out, Quad cores are already cheap. It's therefore not anymore a case of waiting for them to come down in price, they're already pretty affordable. If your budget is x, and you can get either a dual core or a quad core for about x, then it makes all the sense in the world to buy the quad. The only time it would make sense to go dual, is if you only want to spend up to x/2 on a dual core. Otherwise you're actually paying more for getting less (in a cost per core/cost per cpu cycle sense.) In reality, we have the following:
Q6600 (4 core, 2.4Ghz stock): ~ £180
Total Number of cycles in a second: ~ 9600 million
Effective Cost for 1000 million cycles: ~ £18.75
E6850 (2 core, 3.0Ghz stock): ~ £176
Total Number of cycles in a second: ~ 6000 million
Effective Cost for 1000 million cycles: ~ £29.33
E4500 (2 core, 2.2Ghz stock): ~ £90
Number of cycles in a second: ~ 4.4Ghz
Effective Cost for 1000 million cycles: ~ £20.45
So in a "price per processing cycle" sense, the quad is the cheapest from the 3 listed, the second best being the E4500, followed lastly by the E6850.
As you can tell by now, my leaning is toward a quad in situations where the CPU budget is around £180 or so, since the single threaded case does not present to me a compelling reason to buy the dual core, given that there's at best only a 10% delta in the overclocked case outlined above, given that the difference in power requirement seems relatively small (particularly compared to the gargantuan requirements of the other components in the system eg the video card etc), and finally given that you're actuallypaying substantially more per Ghz processing capability for many of the dual-cores. So, for smaller budgets/requirements, the closest step down seems to be something like the E4500, which is at least approximately comparable in a cost per Ghz processing capability and would be reasonable.
Right, now I'll go put on my asbestos underwear and wait for the flaming to begin!
