After Effects: http://www.barefeats.com/aecs6.html
If you rely on cpu alone, it may look like it is working, or lagging, or wating, but with the combined power of GPGPU can make all the difference.
Communicating over Quick-Path is okay but Intel knows that they need to up the GT and L3 cache to improve it more. ie, there are times when two 6-core Mac Pros are faster than one 12-core, given the same GHz.
the real reason people bought dual with lower clock was hoping it would do more work, but when you have 3.33GHz a lot of work that cann't be dispatched and is not multi-thread or multicore all benefit.
W3680s are relatively cheap @ $600, you will not find X5600 sets unless you can make your system pay for whatever you throw at it and do so in a year to write it off and pay for doing more projects and more income.
This guy goes kind of crazy with custom setup 12-core 3.33s, 960GB SSD PCIe and 96GB RAM.
More RAM helps some areas but not rendering. So find the bottlenecks or throw everything at the problem and hope.
$1660 a piece X5680 - $1470 on Amazon
http://ark.intel.com/products/47916/Intel-Xeon-Processor-X5680-(12M-Cache-3_33-G Hz-6_40-GTs-Intel-QPI)
http://www.amazon.com/Intel-X5680-Processor-Socket-LGA1366/dp/B003ELYSJQ/
Only if you shop ebay for used parts, and by then whatever Intel and Apple agree on next go round new is a better system solution most of the time than to throw $3000 into an older system. $3k would be another 6-core 3.33 if that helps.