Only purchase the X99 system if you are going to be seriously overclocking it. Unless you are buying an M.2 drive with at least 2 GPUs, extra performance of 16x/16x will not provide more than a 2-3% boost over a Z97 8x/8x setup. Again, if you won't overclock the CPU, you'll end up with a slower system than i7 4790k and dual 880s. If you plan on buying 3x 880s, need to start thinking about a monitor setup which needs that kind of power.
I think for your first system, you should get 4790K and 870s. Since you aren't that familiar with PC parts, as a first time PC buyer you'll have a large inclination to overspend on everything since you'll feel if it's not close to the best, it's not good enough. Just buy the 4790K and learn the basics of overclocking on a $40 Thermalright True Spirit 140. Then you can upgrade at least 2 more times on your GPU side. In 2 years you can always resell these parts and get Skylake/SK-E and GM200 or Pascal/ R9 400s.
Also, 5820/5930K overclocking is an unknown right now. Without having that data on hand, it's impossible to say if it could even reach 4790Ks mild 4.6-4.7 OC.