Can't get past 422 bus speed on my board

Your FSB essentially IS your OC. If you're changing the front bus speed, you're changing how hard your board has to work, that is why it is harder to get a 500x8 OC than it is to get a 400x10. What you may want to try is increasing your NB voltage, it gives a little bit more "umph" to your OC and allows it to get a little higher. Don't make the value more than like 1.35V if it's passively cooled. I would suggest leaving your multiplier as high as it will go, as well.

EDIT: I'm using a DFI DK P35 with an E6750 and it's at 425, no issues. Could also be luck of the draw.

What NB cooling do you have and what are all of your voltages set to?

NB voltage is set to 1.45 volts on my current OC.

My fear is that if I keep this board and setup if Core i7 really can't OC like the tests show (i.e. if the safe voltage is only 1.3625 volts than even the extreme is limited to about 3.8 GHz) than I'm going to swap out my chip for a Q9650. If my board can't get close to 425 bus speed adn above I won't be able to get my chip over 4 GHz, and that is what I really want.

I need to check on my all my voltages though. Let me get back to you guys on that.

Could it also be because I have 4GB of memory. My limiting factor before was that I couldn't get my memory to 1066 which is what it should be at, even at stock speeds.

The rounding error is usually a lack of voltage... happened to my P4 overclock until I raised voltage a little.

I don't want to raise my CPU voltage anymore, so I guess my OC is the best I can do. Still a good OC though.