1) Although I know no one could possibly tell how the future will go, but will 6-core show much more significant upgrade than 4 after years? Especially with this 3930k, or it will remain the same difference in performance ?
You know what, this is a tough area. There's no doubt that a few years down the line programs will be coded for 6 cores and you will definitely see an improvement with the 3930k over the 3770k. How much of an improvement? Nonody knows for sure, but by this you will have to be the judge, do you believe in complete future proofing and spending the extra money even though you will not see individual results or just opting for cheaper and still top of the line performance, until 6 cores are supported more.
2) How about the differences on 3D rendering? (670sli - 2,688 cuda vs 680sli/690 - 3,072 cuda) Does anyone know whether it is a huge difference or minor?
To be honest, I don't think the extra cuda cores will make that big of a difference if your 3d renderer is mainly CPU intensive. Not all of the cores will be used up in that respect, so I can't imagine the 680 would be that much better than a 670.
3) Thanks, how about 2700k or 3770k? they are about the same price, would it be worthy to get the 3770k? for 2700k i will be looking to OC to 4.5ghz, and 3770k perhaps 4.2 ghz, and from my research 2700k seems to be a better one?
3770k are made with a new manufacturing process and in a nutshell, the 3xxx series should overclock cooler -- on paper that is. But benchmarks have showed the the 3xxx series GPUs so heat up quite a bit even under their new architecture. Though, their temps will be just abou the same as 2xxx series CPUs. So just go for 3770k, it has intel hd graphics 4000, to help boost video rendering. The 2770k doesn't have hd 4000
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