The extra cores
already matter today in Crysis 3, Ryse Son of Rome, GTA V, The Witcher 3 if you have enough GPU firepower. If he wants to keep his rig for 4-5 years, there will be even more games that use extra cores with DX12.
Also, if the user ever intends to do things outside of gaming/
multi-task, 6-8 cores will be faster than the mainstream i7.
If he isn't going to be overclocking, i7-6700K > 5820K.
When you say "better mobo" are you looking for specific features? I mean the Hero and Deluxe motherboards have a bunch of marketing features that cost large premiums that unless you plan on using them specifically are just money wasted. You can get a very good Z170 board for less $.
http://www.newegg.com/Product/Produ...cm_re=z170_motherboard-_-13-157-633-_-Product
http://www.newegg.com/Product/Produ...cm_re=z170_motherboard-_-13-130-867-_-Product
http://www.newegg.com/Product/Produ...cm_re=z170_motherboard-_-13-132-567-_-Product
http://www.newegg.com/Product/Produ...cm_re=z170_motherboard-_-13-157-629-_-Product
Asus boards are good but it doesn't mean they are automatically worth paying large premiums for over Gigabyte, MSI, Asrock. Just as an example, ASUS Pro Clock allows BCLK overclocking to 400Mhz but Asrock's
$150 board is rated up to 650Mhz. It makes sense to research if you actually need the features on the board or otherwise you are just paying extra for little benefit.
Do you have another CPU to try in your X58 platform? You could try dropping a
Xeon 56xx into your system and wait it out until Skylake-E. Since you keep your systems for so long, it's something to consider (unless your mobo is faulty).
Honestly if you only aiming for 4.0-4.2Ghz overclock on the 5820K, might as well get the i7 6700K because without overclocking it'll do 4.2Ghz which seems you'd be happy with. But considering the $350 price of the i7-6700K, maybe just pay
$419 for the 5820K @ 4.6Ghz?
46x CPU Multiplier
1.328V CPU VCORE (Or less)
1.95V CPU INPUT VOLTAGE (Or less)
Test equipment:
Motherboard: Asus X99 Deluxe (this is more or less similar to the X99-A board you want to get). If you are buying the X99-A board, make sure to get the one with
USB 3.1 as it costs just a bit extra.