Hi fellows ! 

I'm thinking about this PC build : 

Sabertooth Z97 mk2

i7 4790

DDR3 2x8Gb 

MSI GTX970 Gaming  4G

128Gb Crucial MX100 SSD Primary drive for windows

500Gb Crucial MX100 SSD for Steam

Obviously HDD, and some other stuff ^^

Any advice would be much appreciated, I'm aiming to run The Witcher 3 without a breeze

My advice: buy PS4.

- M, Carl

Well, that PC would certainly run it like cray cray but imo, it would be better if you were to wait a little longer before buying a new PC. The 8gb version of the 970/980 series should be released this year, DDR4 should also be released this year and granted the difference isn't zomfg compared to DDR3 but for future upgradability purposes, I highly recommend waiting for DDR4. And USB 3.1 should also be released this year and it has twice the throughput as USB 3.0!

So it would be really great if you could wait just a little longer for even more of a kickass PC

Seems good, but I would get rid of the 3 drives you have and just get 1 x TB SSD.  Also consider an R9 290.  It's just as strong for a lot cheaper, and the power use isn't as much as some make it seem.  Both are good choices though.

Looks fine but will certainly not run The Witcher 1080p60 on ultra. Expect to play it on high settings with maybe some effects toned down to achieve stable 60fps.

Absolutely nothing wrong with this build though.

Well the 8GB version of the 290X is already out for $70 less than a 4GB 980.  Also the 390X will be out within half a year and should be quite a bit more powerful than the 980...

Well there you go! More reason to wait!

Depends a little on what parts you already have. I assume you have a computer atm? An SSD is not necessary for games. It won't do anything for how the game looks. So I say for now, skip the 500 SSD, buy a 256 GB SSD for OS and other programs, and a large SATA-drive for games etc.

Sorry, but that's just nonsensical. Not only is DDR4 currently a bad idea since it's actually slower than DDR3 in some scenarios it will literally not make a difference at all. I also don't know what you would achieve with 8GB of VRAM on a single gpu. By the time 4GB aren't enough anymore it's time for a new GPU anyway.

The money saved by not getting DDR4 and overpriced GPUs could be put to better use in an upgrade from 970 to 980 which will actually make a big difference.

In 4K the 8GB version of the 290X beat the 4GB version by ~30% in some games.  It breezed right past the 980 in those benchmarks as well.  So already some games utilize 8GB.