To SLI or not to SLI...

So the setup is this:

CPU: Phenom II x6 1090T @ 3.2Ghz
Motherboard: Asus Crosshair IV Formula
Memory: 8gb
HD1: Samsung 840 Pro 256
HD2: WD 1TB Black (FAEX)
GPU: Sapphire ATI Radeon 58xx (highest single GPU)
PSU: Thermaltake 850w

I'm replacing the 58xx with a 780ti (reference for future watercooling) and in about 9 months am doing a complete rebuild down to the case. (ok, the 840 pro is migrating , but that's it!)

It will look like:

CPU: i7-4790K
Motherboard: EVGA Classified Z97
Memory: 32gb
HD1: Samsung 840 Pro 256 (same drive, will probably upgrade to the 850 pro 512gb at some point)
HD2: WD 4TB Black (FZEX)
GPU1: EVGA reference 780ti
GPU2: EVGA reference 780ti
PSU: Corsair AX1200i (only because I will need a bit more than the AX860i provides and I really like the link feature)

Given that I desire to do 4k gaming (single screen) my question is one of performance of card #3. That particular MB can handle x16/x16 but if I add a third card, the PCI slots drop to 8x/8x/8x. Is 3 cards @ 8x better than 2 cards @ 16x and will I be able to go nuts with the settings @ 4k with just 2 cards?

As you might have guessed, money isn't the primary concern here, but it does factor in by possibly extending (or reducing) the time frame.

You're looking at very minimal gains comparing 8x to 16x. I wouldn't even consider it, honestly. Any reason you're not considering a 980 instead of the 780ti?

chill, 9 months the landscape will change significantly

Broadwell
980Ti - 20nm ?
M.2 gains
DDR4 possibly near or at 4K speeds
New gen boards

If you want better graphics performance now, though, I think buying a good single gpu, then adding another later is a good plan.

I'm in a time crunch, my favorite game just updated their system reqs (effective new years; ya, they're nice with the lead time, they gave 6mo) and I need a new card. From what I had read, the 780ti was slightly better than the 980, but now I'm hearing otherwise, so the 980 is back in the running.

edit: ok, the news that nvidia will be discontinuing the 780ti chip production (if true) kinda makes my choice for me, doesn't it? (at least if I want to sli later and not now)

As to the other boards coming out, I hope they do. It'll drop the price of the Z97 boards. I'm not looking at a truly bleeding edge computer, but a step back from it. Besides, all things are subject to change until the "submit order" button is clicked.

Just to clarify though: 8x vs 16x is irrelevant, it's simply [more cards is better]?