The Intel E6600 is 2.4GHz x2, has 32k+32k of level 1 cache, and 4mb of level 2 cache.
The Intel QX6700 is 2.66Ghz x4, has 64k+64k of level 1 cache, and 8mb of level 2 cache.  They are really, really not the same animal at all.  

Also note that if you can get the OEM version of the chip you should do so and save yourself a couple of hundred bucks - when you buy the retail version, you're mostly paying extra for the fan , which is trash.
http://www.tigerdirect.com/applications/searchtools/item-Details.asp?EdpNo=2573616&sku=CP2-DUO-QX6700

In cases where all of your applications' instructions will entirely fit in L1 and L2 cache where appropriate, then having gobs more will not be of great benefit; but it really does make an enormous difference in some sitations.  Also throwing another two cores at your app, if the app can take advantage of it, will be a pretty much linear improvement in speed for the really serious stuff e.g. Vue or 3ds Max. 

Consider one set of benchmarks:
http://www.extremetech.com/article2/0,1697,2049694,00.asp

The numbers pretty much speak for themselves, really, although that is only one benchmarker.  Do you sit and stare at your machine(s) while rendering is going on?  how much money is it worth to you to reduce the time you spend sitting and staring?  the answer won't be the same for everybody.
Filter out the 3dmark stats and the like, which are as much about graphics card, and look at the 3dsmax and povray numbers.

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