I can give you an answer concerning spi @450 & spi@500!
If you look at the screen fsb@450, you can notice that the x6800@3150 (crystalcpuid) (ratio 7x used)! The speedstep beeing enabled, at rest cpuz report 6x!
On the screen fsb@500, I locked the cpu ratio @6x in bios, so x6800@3Ghz!
So lower...