While synthetic benchmarks are not the "real thing" they give quite a good impression of a computer's performance. The general rule is the higher the score, the better.

3DMark 01 SE

3DMark 2001 sees a little boost from four cores. Considering that the benchmark is five years old and does not support multi-core CPUs, this is still better than nothing.

3DMark 03

In 3DMark 03 we get to see a new type of test - CPU. While the increase in the overall score from two to four cores (E6700 to QX6700) is very close to zero, the CPU test shows a greater spread between the two processors.

3DMark 05

3DMark 06

The latest incarnation of 3DMark is multi-threaded, and clearly shows the benefit of having a quad-core CPU. The overclocked QX6700 has almost 2x the score in the CPU test compared to the E6700, which only has two cores.

3DMark does not yet incorporate the CPU score into the overall score, but the 2007 version may do so. This is quite a debated issue, as some think that 3DMark is becoming too CPU dependent.

PCMark 05

PCMark, unlike the 3DMark series, attempts to analyze the whole computer in terms of performance. Therefore, it incorporates the CPU score into the final result. If we consider the overall score, the QX6700 has an advantage of about ~1000 points over its brother, the E6700. We can spot that each 266 MHz, the score increases by around 400 points (E6400->E6600->E6700). To gain an increase comparable to going quad core, we would theoretically have to add ~660MHz to the clock speed of the E6700 (which would mean a final clock speed of 3.3 GHz).

The CPU score shows a 700 point increase for every 266 MHz added. Going from two to four cores, while maintaining the same internal clock speed, we see a boost of 1700 points.

Aquamark

Science mark

Cinebench

Cinebench clearly shows the performance advantage of four CPUs. It is almost as if you literally added another E6700 to your system.