Firstly, thanks for the replies guys! :)

Budget-wise, I'm not really sure. I can afford to go nuts but at the same time, i'd feel bad if I simply oversped. What's a good number? I'm assuming an upgrade cycle of every 3-4 years. Let's say 3 grand? What about 5?

That's quite a big budget!

If you're mainly going to use it for gaming, then I'd forget about super high end CPUs like Intel's LGA2011v3. They are not worth it and with DX12 and Vulkan coming in the future, you'll ran out of GPU horsepower before the CPU becomes a bottleneck.

And instead of mentioning specific hardware, I'll point you towards to a couple of System Guides so you can see what the professionals suggest and let you make your choice:

TheTechReport: TR's February 2015 System Guide

Tom's Hardware: System Builder Marathon Q1 2015: $1750 Performance PC and it's alternative.

I'd get an aditional external HDD (2 or 4TB) and use it to make your backups.

Probably my current rig with 2 980s and a 4K monitor

edit: here's my current build

Processor: Intel i7 4770K Quad Core

Memory: Corsair Vengeance 16GB (2x8GB) DDR3 1600 MHz

Motherboard: Asus Z87 PRO LGA 1150

Power Supply: FirePower Silencer MK III 750W

Video Card: Asus GeForce GTX770 2GB GDDR5

Blu-ray burner: ASUS 12X Blu-ray Internal Burner Drive with Disc Encryption

SSD: Samsung 840 Pro Series 2.5 Inch Solid State Drive, 256 GB

Hard Drive: Western Digital Caviar Black 2 TB SATA III 7200 RPM 64 MB

Sound Card:  ASUS PCI-Express x1 7.1 Channel Sound Card

Speakers: Logitech Z623 200 Watt Speaker System

OS: Windows 8.1 Pro

Case: NZXT Technologies H2 Classic Silent Midtower Chassis

Mouse: Razer Naga

Keyboard: Razer Deathstalker

Headset: Corsair Vengeance 2100 Wireless 7.1 

JEMC is right, you could get better/near the same performance for a couple grand less... 

Yeah, you should probably actually read what I wrote.  Instead of just looking at the prices on the picture.

Quick question, I notice guides recommend ddr3. Is there a benefit to going ddr4? Should I get the faster ddr3? What's the benefit of faster memory in terms of gaming? Or is that just for video editing abs such?

Should I get the titan or sli 2x 980's? Is that enough for 4k gaming? Or is 2 titans the min? What kinda power supply do i need for 2 sli'd 980's? What manufacterer is best for this?

Also, how does sli work? I've done it before but I've always been told to use the exact same vid cards for sli.

Does it have to be the exact same model gfx card? Like the exact same manufacterer/model of gtx 980's? Or can I have an overclocked gigabyte gtx980 and one regular msi one? Can i do 1x 980 and 1x 970?

Intel's s1150 and AMD CPUs and motherboards are only compatible with DDR3 memory. You have to go with Intel and its socket i7-5xx0 processors from the 2011v3 socket to use DDR4 and a few months ago there was an user here that downgraded from the new i7-5xx0 to the old s.2011 i7-4xx0 because DDR4 was slower in games.

You'll be better with a high end s1150 processor with the fastest RAM you can find.

2 980 in SLI are faster than a Titan X, which is almost but not quite enough to game at 4K at 60fps and it will only get worse as new and more demanding games like The Witcher 3 are launched.

I'd go with at least a good 850W PSU. Corsair, SilverStone, Seasonic are good brands.

As for SLI, I've never used it (I prefer to go with a single card to avoid those kind of problems), but you'll need to use two identical cards from the same brand to make it work.

What are the best manufacterers for the 980gtx? Are all 980's SLI-enabled? Checking amazon and I only see evga as the only one that specifies SLI. :)

What's the best ddr3 memory I can find?

Again, thanks for the replies guys.

If you're planning on building a 4k rig I would wait for cards with more than 4gb of ram.I game at 4k with 3xgtx780ti and I always have to turn down textures, sometimes even shadows, the 780ti is a 3gb card though.Back when I built the rig in 2013 3gb was more than enough to max everything at 4k, nowdays some games use more than 4gb even at 1080p.

Now if you still want to build that rig,and money is not a problem, I suggest you get 3 EVGA gtx 980 classiffied.Only 2 980 are not enough if you want to a completely smooth experience with no frame drops whatsoever.

For cpu and again only if  money is not a problem get an i7 5960X but only if you aren't planning on upgrading in a few year later, if you're planning on upgrading later, get an i7-5930K.

For ram is the same get ddr4 if you aren't upgrading, and ddr3 if you are upgrading later.