sai0449 :
But what will be the best. Config for desktop comparatively with Asus rog g751(16gb ram .I7)
What's your max budget, and do you need just the main components and case, or keyboard, mouse, display, and operating system as well? Also, do you feel confident in assembling the parts or have a friend you trust that can? There are many good tutorial videos on YouTube about how to build a PC.
There ARE graphics cards now that can play most games at 4k without using a multi card setup. Either way it's fairly expensive, but with a single high end card vs 2 that aren't top tier, you get more video memory. The exception is AMD's 300 series, which are not as powerful as the 980 Ti for instance, but have 8GB VRAM.
You can get one 980 Ti for $600, and it's a good way to go for 4k IMO. There will be a few games like AC Unity and Crysis 3 that you'll need to either drop settings a bit or run at a lower res like 1440p, but going forward, you'll have best future readiness over other options. You can always add another 980 Ti on down the road, or sell it when Pascal comes along and stick with single GPU and avoid SLI problems that can pop up.
As far as CPU and platform in general, many are skipping the more expensive Skylake 1151 platform and DDR4, which don't really offer any performance gain over Haswell. Best bang for buck would be an 1150 MB with 4690k CPU. You'll get higher clock speed without an OC on a 4790k, but for $100 more. OCing a 4690k is pretty easy too, but you'll need to spend at least $30 of that $100 you save on a CPU cooler to OC it.
980 Ti benched on 21 games @ 4k
https://www.techpowerup.com/reviews/NVIDIA/GeForce_GTX_980_Ti/1.html
980 Ti cards currently going for $600
(EVGA card has free game and better warranty)
http://www.newegg.com/Product/Product.aspx?Item=N82E16814487142&utm_medium=Email&utm_source=IGNEFL102015&cm_mmc=EMC-IGNEFL102015-_-EMC-102015-Latest-_-DesktopGraphicsCards-_-14487142-S0C
http://www.newegg.com/Product/Product.aspx?Item=N82E16814127902&cm_re=msi_980_ti-_-14-127-902-_-Product