Intel Celeron 420 3.0 GHz VS Intel Core 2 Duo 1.0 GHz

The C2D will be marginally faster at that clock. Why do you have it @1ghz BTW? Are you using it for a HTPC?

Probably a wash between the two. I'd argue for the celerons efficiency but goes out the window with such a ridiculous overclock. I'd go core2duo , just cause it has 2 cores. Mean both (assuming 2007) have the same cache and FSB so performance will be very similar on paper at least.

The Celeron 420 will undeniably be quite a bit faster.

It is running at three times the clock speed of the underclocked Core 2 Duo but uses the same microarchitecture. There is a moderate improvement in desktop application performance with increasing cache size- I am guessing roughly 10-15% tops based on performance of the 1 MB L2 Pentium Dual Cores vs. the 2 MB Core 2 Duos. Multithreaded tasks generally only scale up to 75-85% anyway so at the best case, the 1.0 GHz Allendale (native 2 MB L2 65 nm desktop Core 2 Duo = Allendale) will be as fast as a 2.0 GHz Conroe-L.

Also consider how you arrive at those clock speeds. An Allendale generally runs a 200 MHz (800 MHz FSB) or a 266 MHz (1066 MHz FSB) clock speed and it has unlocked multipliers from 6x on up to the maximum one for its rated clock speed for SpeedStep to work. Celerons have locked multipliers and no SpeedStep. The Celeron 420 has 200 MHz clock and an x8 multiplier. Normal FSB clock speeds on desktop Core 2-derived chips are 133 MHz (533 FSB), 200 MHz (800 FSB), 266 MHz (1066 FSB), and 333 MHz (333 MHz.) Normal supported RAM clock speeds for that generation are 133 MHz (DDR2-533 and DDR3-1066), 166 MHz (DDR3-667), and 200 MHz (DDR2-800.) Some performance 30-series units might support DDR3-1333 (166 MHz) or DDR2-1066 (266 MHz).

So, your Celeron 420 running at 3.0 GHz requires a 375 MHz FSB clock (1500 FSB.) At 3.2 GHz it would run at an even 400 MHz. A 375 MHz FSB clock would likely be using straps suitable a 333 MHz stock FSB unit, so your RAM would be slightly overclocked, giving better performance. At 3.2 GHz this will be even more pronounced. Overclocked RAM tends to perform a little better than standard clocked RAM until you get to the hairy edge of stability and have to loosen up timings a lot.

You also can't underclock your Allendale to 1.0 GHz without cutting your FSB clock. The minimum clock speed with a stock FSB on a C2D E6000 series would be 1.60 GHz (266x6) and on an E4000 series would be 1.20 GHz (200x6.) You need a 166 MHz clock to be at 1.00 GHz. A 166 MHz FSB clock has never been used on an Intel desktop chip and you would either have to use memory straps for a 533 FSB chip and overclock the RAM (slightly better performance) or use straps for an 800 FSB and underclock the RAM (lower performance.)

So, it's a little more complex than what it originally appears, but the Celeron is clearly faster.

The first thing I wrote was your answer- the Celeron will be considerably faster, likely in the neighborhood of about a third faster in well multithreaded tasks and around 3x as fast in single-threaded stuff. It will be program dependent. Why don't you pick up a C2D E4300 or E4400 off eBay and bench it yourself at 1.0 GHz, they are only about $10 apiece. That would be a cheap way to decisively end the debate.