If your motherboard operates like most what happens is the northbridge has a divider circuit and changes it's own multiplier at every even multiple of 66MHz. If your chip is designed for 266 MHz it will operate best at 333 or 400MHz. It may not operate at all from say 450 to 465 MHz, and then start working again at 466MHz.
You can also lower the multiplier and adjust FSB that you feel is best. My E6400 (266 * 8 ) total 2.13GHz, I run it at 7 * 400 for 2.8. It will run at 8 * 400 fine with extra voltage, which I would rather not do. I don't even have any heat issues, it's 65nm.
I would think you should be able to run 8 * 400 at stock voltage.
For stability and accuracy you need to run Prime 95 on each processor, so for you that would be 2, and I think there's a setting for which processor.
If you left that C1E option on (or whatever it's called in the BIOS) the processor may lower it's multiplier to slow down and use less power when it's idle. The makes for very strange results in CPUZ. All you need to do is to move the mouse real fast and the multiplier will change right before your eyes.