How much faster is dual Xeon 5680 vs single 5680 for multithreaded applications

[idata]

I know that the QPI link speed limitations means that dual Xeon 5680s will not run twice as fast as a single. So, for multithreaded applications that can use as many threads as you can throw at it what would be the gain for having dual Xeon 5680 versus single?

Very simply if I have an application that will distribute it's workload accross as many threads or computers I can give it. So I debating about building:

1) 1 x Dual Xeon 5680 System

2) 2 x Single Xeon 5680 Systems

3) 2 x I7-980X systems

I know that the limiting factor would be the size of the data being passed over the network, but if the packets are small which option do you think is best? (Money is not an option).

[idata]

Well, I have somehow the same question.

Let say on pure CPU application such as video editing or a game like FSX which is a rare game based on CPU and where the graphic card is the hot item to have.

Is there a rule of thumb where a dual quadcore PC will be faster than a single faster quadcore. just as an exemple, dual i7-860 vs i7-980?

Stephane