Best Rig for Budget + Use

What monitor resolutions are you using ?  That makes all the difference in a video card selection.

I don't have any experience on UE development, but for gaming, the Z170 is a newer platform and faster single cores and better for games.  The added cores in X99 wouldn't be utilized.

Ram at 2666 is the sweet spot. 

M.2 SSD is many times faster than SATA restricted drives.  Not to say the SATA SSD isn't fast, it sure is, but Pcie buss is like much faster.

My pick is the EVGA card for its excellent support, plus the ACX 3.0 has zero rpm fans, meaning at low load and below about 65 C, the fans stay off. 

The Vanquish is best bang for your buck, but you don't get to choose the power supply, cooling, etc.   If you want an sli setup for  4K gaming, you'll want to go custom with Apollo, Velox, etc.

Hope this helps and good luck.

The Velox has the definite edge in cooling.  It has the 3x front and 2x top fans vs the 2x front in Apollo.  For a single card setup either will work, but if you go sli, which is needed for max gaming on a 4 K monitor, the Velox would be my pick to help get the heat from the video cards out of the case.  

Problem is, a sli, 2x gtx 1080, will go way over your budget.  A single 1080 will play on a 4K, but not maxed out graphics and maintain a decent frame rate.  Turning the graphics down to high and other tweaks will still look good and play well, but everyone seems to want to max out everything.

I haven't shown you a config yet because i'm not sure how many cards you want.

What the heck        Here's a Velox build for 4K with sli 1080 and a good maintenance free cpu cooler. 

Cost $3912   Config # 1518606

Chassis Model: Special Deal Hot Seller - Digital Storm Velox
Exterior Finish: - Standard Factory Finish
Trim Accents: - Standard Factory Finish
Processor: Intel Core i7 6700K 4.0GHz (Codename Skylake) (Unlocked CPU) (Quad Core)
Motherboard: ASUS Z170-PRO GAMING (Intel Z170 Chipset) (Up to 5x PCI-E Devices)
System Memory: 16GB DDR4 2666MHz Digital Storm Performance Series
Power Supply: 1000W EVGA 1000 GQ <br><strong></strong>
Expansion Bay: Option Not Available
Optical Drive: DVD-R/RW/CD-R/RW (DVD Writer 8x / CD-Writer 8x) (Internal)
Storage Set 1: 1x SSD (500GB Samsung 850 EVO) <br><strong></strong>
Storage Set 2: 1x Storage (1TB Western Digital - Black Edition) <br><strong></strong>
Storage Set 3: - No Thanks
RAID Config: - No Thanks
RAID Card: - No Thanks
Internet Access: High Speed Network Port (Supports High-Speed Cable / DSL / Network Connections)
Graphics Card(s): 2x SLI Dual (GeForce GTX 1080 8GB (EVGA ACX 3.0) (VR Ready) <br><strong></strong>
Sound Card: Integrated Motherboard Audio
HPC Processor: - No Thanks
Extreme Cooling: H20: Stage 2: Corsair H115i - 280mm Liquid CPU Cooler (Fully Sealed + No Maintenance) <br><strong></strong>
HydroLux Tubing Style: - Not Applicable, I do not have a custom HydroLux liquid cooling system selected
HydroLux Fluid Color: - Not Applicable, I do not have a custom HydroLux liquid cooling system selected
Cable Management: Premium Cable Management (Strategically Routed & Organized for Airflow)
Chassis Fans: Standard Factory Chassis Fans
Internal Lighting: - No Thanks
Airflow Control: - No Thanks
Chassis Mods: Fan Blow-Hole: Graphics Card Area Only: Laser Cut and Mount (1x) Side Window Fan <br><strong></strong>
Noise Reduction: - No Thanks
LaserMark: - No Thanks
Boost Processor: Stage 1: Overclock CPU - Up to 4.4GHz (Depends on Cooling and Motherboard)
Boost Graphics Card(s): - No Thanks, Please do not overclock my video card(s)
Boost OS: - No Thanks, Please do not tweak the services on the operating system
Windows OS: Microsoft Windows 10 Home (64-Bit Edition)
Recovery Tools: Factory Reset Feature (Restore Windows + Drivers) (Partition up to 25GB of Storage)
Virus Protection: FREE: McAfee AntiVirus Plus (1 Year Service Activation Card) (Not Pre-installed) ($35 Value)
Office: - No Thanks
Game: - No Thanks
Display: - No Thanks
Surge Shield: - No Thanks
Speakers: - No Thanks
Keyboard: - No Thanks
Mouse: - No Thanks
Branded Gear: - No Thanks
Priority Build: - No Thanks, Ship Within 10-15 Business Days After Order Is Successfully Processed
Warranty: Life-time Expert Care with 3 Year Limited Warranty (3 Year Labor & 1 Year Part Replacement)

I put in the side panel blow-hole with fan to help kick the graphics cards heat out of the case.   The ACX 3.0, vs the Asus and Founders, does dump the heat into the case.  

If you don't want sli, then drop the 2nd card and drop the psu to a 750watt and drop the blow-hole.

I'm still using my 5 year old rig with an overclocked 2500k, 8gb ram and gtx 970. Works great on my Dell 1080p UltraSharp.   Getting close to needing or wanting something newer to play with.

Adding another card is just plugging it in, plug in the psu cables and slip on the sli bridge.

I haven't checked out fans in awhile. What your getting here is just fine, although there are a ton of fans to choose from with better flow, quieter, etc.

I think stage 1 0c is a good pick. The difference between a 4.4 and 4.6 clock isn't very noticeable.    DS is conservative in setting oc because they want stability and less problems.

Most memory is very good. They have been using ADATA ram. Added cost for ram is if you want to oc it, a taller heat spreader which is only for higher clock, or a brand you trust.