Just for the processor? I'm asking because that's enough to buy an AMD CPU + 8GB RAM + a mainboard + powersupply to get 70% of the multicore performance of a intel 6700k (again the cpu costs more than this setup).

Intel and AMD are market leaders in raw price/performance. They drop mad money on custom designs at cutting-edge nodes to do it, too. The Cavium chips might be custom but probably standard cell. Cost way less. Further, you get 16 RISC cores with no x86 baggage, they run Linux, PCI form factor, and low power. Onboard are usually accelerators for TCP/IP, compression, encryption, and so on that run in Gbps that your processor cores don't have to handle. The CPU cores themselves run pretty fast.

The hybrid seemed like one of the best designs. Probably why AMD started copying it with their semi-custom business followed by Intel. But those are still full-on x86 processors with all kinds of circuits you probably don't need plus maybe some backdoors for AMT, etc. Cavium's design is much more interesting and maybe safer. I've mainly encouraged people to clone it with RISC-V cores rather than straight-up buy them. ;)

The newer ones are pretty impressive in performance and accelerators, though:

https://www.cavium.com/OCTEON-III_CN7XXX.html

I looked at that, but: * AMD no ECC, nor a similar chip with it. Intel has a Xeon E3 which is basically core as the 6700 with ECC enabled. Doesn't hurt that it's cheaper as well. * AMD had 70% of the throughput, but much less on anything single threaded. So many normal UI apps like say web browsing are slower. * the AMD solution runs hotter, so you need to either tolerate more noise or pay more for case/fans/power supplies * My use case includes running 24/7 so burning extra watts (and running the AC to cool them) adds up.

After considering it all I went with the E3-1230v5. Near 6700k multicore performance, near 6700k single thread performance, max 80 watt TDP, and idles much lower. I also get a M.2 slot which allows pretty amazing performance (well above SATA) and a tiny footprint (gum stick or so).

I don't buy systems often though, the machine it's replacing is a quad core 3.0 GHz machines (Q6600) with 8GB ram from 2007. So I don't mind spending a bit more if it's going to last. Hopefully with 32GB ram this new machine will last quite a while.

My machine is about as old given it's a Core Duo 2 with 4GB of RAM. One of the Dell laptops designed for Ubuntu. What do you use yours for where a quad-core 3GHz and 8GB RAM isn't enough now but was a while back? Just curious.

AFAIK, all modern AMD CPUs support ECC. You just need motherboard support.

That was true on the AM3+ Processors with the 890FX and 990FX chipsets. I do not believe it is the case on AMD's modern CPU sockets, but I would be happy to be shown to be wrong.