As I understand it:
The northbridge/Memory Controler Hub(MCH) has a set of straps it runs at. The lower the strap the better the performance. These straps are set up for verious FSB ranges but are not neccesarily used for the range they're designed for.
The Northbridge has it's own clock so it can run on a devider from the FSB, I'm not sure how much of a performance impact this has. I also believe running the Northbridge at higher clocks improves system performance much like running RAM at higher clocks.
The strap is a set of timings, sort of like memory timings, but has a lot more impact on system performance than memory timings.
There isn't a lot of control for overclockers with regards to the strap settings or northbridge clock but I think the 975X chipsets give you a little more control than the 965P.
This artical looks quite good:
www.thetechrepository.com/showthread.php?t=30
The impact:
When you go past 400Mhz FSB on a 965 chipset you hit the 1333 strap which slows the system down. There was a thread on here to do with decreased performance between 2.8Ghz and 3.6Ghz or something with the E6300 - I'll look it up and add link on edit. Conversely below 267fsb I think the P965 runs on a devider - so the northbridge runs faster than the FSB, which means at stock the E4300 has a devider which you only lose after overclocking past 2.4Ghz(ish).
Having said all this I believe the E6400 will hit the strap change at 3.2Ghz.
Edit:
forums.overclockers.co.uk/showthread.php?t=17675889&highlight=2.8Ghz+strap (link to performance dip thread)
Can anyone confirm all this?