The short end, not much.
The hyper transport and NB speeds are related to each other. To raise HT, NB has to match.
The job of the north bridge is to provide a connection of the processor to the rest of your I/O of various buses and speeds, from PCI to RAM, and HT is part of that connection. HT is also involved when cores reach into cache, or each other's cache.
Since the job of the HT/NB combination is to provide a coupler to the various buses of different speeds, the whole idea is that the CPU's speed is independent of that, and so it hardly makes much difference.
Obviously if HT and NB are run faster, we'd expect some kind of performance increase, but you have to really look for it to notice it, and it's not going to be obvious in many cases.
If the stock HT/NB speed can saturation your RAM speed already, increases in HT/NB aren't doing to be noticeable.
However, in a narrowly focused set of benchmarks, you can notice a minor increase in speeds associated with cache operation or inter-core performance, but it's quite minor and has little impact overall.
There MIGHT be an occasion where lowering HT/NB could let you raise the clock using FSB, if otherwise the HT/NB would become unstable, and just maybe there's an extract couple of MHZ that might fall into place because of an odd phase relationship between the different operating speeds of HT/NB to CPU, but I doubt you'd be able to pick up on it easily.
Obviously jumping from HT 1 to HT 2 and upwards can have a significant impact on overall I/O bandwidth, but moving from 2Ghz to 2.2 or 2.4Ghz isn't generally recognizable, though it does offer something when operations are I/O bound, and may be more noticeable with very high speed RAM overclocks.
Edited by JVene - 5/9/10 at 3:29pm