To be more precise, through HTT base frequency, from which all other frequencies are derived.

The main limiting factor in Nehalem overclocking may be the same as it's main advantage over Core2 arch. - triple channel IMC. Increased number of wires required for 3-channel IMC may negatively affect over clocking headroom.

Lets wait and see. This should be the most exciting upgrade for me since i built my dual dual-core Opteron machine 3 years ago (i was not really impressed by my 8-core Intel Xeon build )