PC Mark's System Test gives little advantage to the four-core Core 2 Extreme QX6850, but it's still an advantage that goes against the Core 2 Duo E6750's higher clock speed.
Though the Core 2 Extreme QX6850 trounces the Core 2 Duo E6750 in PC Mark 2005's CPU test, we were actually expecting the difference to be significantly larger.
PC Mark 2005's memory benchmark appears to show that four-core processors are able to use Intel's high-speed FSB more effectively, at least when CPU speed is pushed to the limit. The Core 2 Duo E6750's memory speed was higher, yet it received a lower score in this test.
Sandra 2005's CPU math test shows the gains we were expecting in PC Mark's CPU test, with an approximate 100% performance improvement for the Core 2 Extreme QX6850's additional cores.