For whatever reason (which is beyond the scope of this thread for now), it seems the 6 core is the sweet spot for macs at the moment.
It is not listed on the mac site, but you can select the 4 core, and upgrade the CPU. Everyone who I know who is using this model is doing MUCH more with their machines than I can with even my 2009 2x4core mac.
Sell the stock ram, and put in at least 12GB if you are using loads of sample based instruments (music production…)
Even when running in a 32 bit environment. Kontakt uses its own memory server (outside the memory nuendo uses) so I frequently use more than 12GB of the 16GB I have installed on my main machine. Vienna Ensemble can also help with memory management of heavy VSTi sessions. There are loads more options around today than there was even 12 months ago.
Just don’t buy the ram from apple if you value your wallet.
The 2x4 and 2x6 models for the moment do not seem to scale well compared to the single chip solutions. For the premium prices charged for these models, it does not seem worth while buying them unless you are also using the mac for other purposes.