We received from Intel a Core 2 Duo E6750 engineering sample, which is basically a Core 2 Duo E6700 with a 1,333 MHz FSB instead of 1,066 MHz. It will be launched later this summer together with several other Core 2 Duo models with the new 1,333 MHz FSB, and from the naming used on E6750 we can assume that Intel will use the number “50” on their model numbers to indicate the new external clock rate. Since we also had available a Core 2 Duo E6700 and a Core 2 Extreme QX6700 in our lab, we could make a terrific comparison between these three CPUs – as all three run internally at 2.66 GHz – to answer two basic questions: By how much the new 1,333 MHz FSB will improve PC performance? What is better, a quad-core CPU with 1,066 MHz FSB or a dual-core CPU with 1,333 MHz FSB? Read on.

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Figure 1: Core 2 Duo E6750 engineering sample.

The only difference between Core 2 Duo E6750 and Core 2 Duo E6700 is really the external clock rate: 1,333 MHz vs. 1,066 MHz. All other specs are the same, like the 4 MB L2 memory cache.

This new external bus works at 333 MHz transferring four data chunks per clock cycle, and that is why it is referred as 1,333 MHz (333 MHz x 4). In reality it doesn’t work at 1,333 MHz.

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Figure 2: Core 2 Duo E6750 specs.

Because this CPU is based on the new 1,333 MHz FSB, it requires a motherboard supporting this new FSB. So you won’t be able to install this or any other CPU based on the new 1,333 MHz external bus on older socket 775 motherboards, meaning that you probably won’t be able to upgrade your CPU with a new one by just replacing the processor, you will probably need to replace your motherboard as well.

Intel P35 and nVidia nForce 680i or 650i chipsets are some of the chipsets that support the new 1,333 MHz FSB. In our review we used a MSI P35 Platinum motherboard, which is based on the new Intel P35 chipset, but we faced a problem with this motherboard during our tests that we need to explain.

We were using DDR2-1066/PC2-8500 memories and we tried to keep them always running at 1,066 MHz, however this was only possible when the CPU external clock was of 1,066 MHz. With other clock rates the motherboard didn’t provide a memory clock multiplier that resulted in 1,066 MHz. With the FSB set at 800 MHz – which was necessary by our Pentium 4 3.4 GHz – the maximum clock rate we could set for our memories was also 800 MHz. For the Core 2 Duo E6750 we had the option to set our memories at 800 MHz, 1000 MHz, 1110 MHz or 1333 MHz. We tried to keep them at 1110 MHz but the system was unstable, thus we set them at 1000 MHz, 66 MHz below the clock rate they should be running at. This slight difference should not impact the final results.

On the table below we summarized below all CPUs included in this review with their main specs. We also added a column called “memory clock” for you to know the clock rate our memories were running when we collected data for each CPU – the clock rate below 1,066 MHz was a limitation from the motherboard we were using.

CPU	Cores	Internal Clock	External Clock	L2 Memory Cache	Platform	TDP	Memory Clock
Core 2 Extreme QX6700	4	2.66 GHz	1,066 MHz (266 MHz x 4)	4 MB x 2	Socket 775	130 W	1,066 MHz
Core 2 Extreme X6800	2	2.93 GHz	1,066 MHz (266 MHz x 4)	4 MB	Socket 775	75 W	1,066 MHz
Core 2 Duo E6750	2	2.66 GHz	1,333 MHz (333 MHz x 4)	4 MB	Socket 775	65 W *	1,000 MHz
Core 2 Duo E6700	2	2.66 GHz	1,066 MHz (266 MHz x 4)	4 MB	Socket 775	65 W	1,066 MHz
Pentium 4 550	1	3.4 GHz	800 MHz (200 MHz x 4)	1 MB	Socket 775	115 W	800 MHz

* To be confirmed.

Unfortunately Intel didn’t provide us a Pentium D or a Pentium Extreme Edition samples for reviewing, and AMD seems to be out of samples and new high-end CPUs, because we haven’t received new samples from them for ages. A pity. If you’d like to see a comparison between the reviewed CPUs and Athlon 64 X2 5000+, please read our Core 2 Extreme QX6700 Review.