This is Octane
So it is all about GPU
At least it is a matter what you can vari and will spent.
I usually do not buy a prebuild system at Dell or whatever - i really want to know whats inside exactly.
So i would start with GPU as described above. A GTX780 with 6GB is a good shot if you can get one.
In actual beta plugin you can define some you your pc memory as out-of-core memory for the renderings so 6Gb will not be your limit for scenes. But using this slows the render down a bit.
16GB of Ram is good. 8 is to less as mentioned. I have no problems to fill 32GB with my DAZ Scenes. So you have to find you way in the middle i guess.
Weather it is DDR3 or DDR4 Ram should not matter. DDR4 and a skylake CPU gives you at the moment no really benefit it seems. If you need to you can go on a more "proved" Hardware to spare some money for the important parts.
As i had learned when you have a large content library i would recommend an SSD or M2 instead a large Harddisk. I mean you will usaully not need 2 TB Harddisk for a Render PC. Only perhaps if you do animations.
But it is really a huge improvement browsing your runtime libs.
Another thing which makes it more comfortable is to have a small videocard for your normal monitor output / display like a GTX650 or something and let the big card only do rendering otherwise with only one card your pc is almost dead when you have an active render running like non paused viewport etc. because the graphics is too busy to draw normal windows.
Also the actual plugin have a slider to vari the GPU Load of Octane the give it more time to do normal operatings like drawing windows but also this slows the render down.
thank you!!!