390x or GTX 980 plus other Qs

Hi,

So I have been using a HD 7950 for 3 years now and it has been a great card for that time but I am thinking its about time for an upgrade, although it does great in modern games its starting to mean that I have to play on low/medium settings to actually get a solid 60fps, games like the Witcher 3 are kind of a no go as its just not smooth and consistant at all. I am thinking about moving up to a 144hz monitor in the next few months so again I kinda need an FPS boost to really take advantage of this.

Now I have always been a more AMD kinda guy since they are usually the cheaper option with similar performance and I looked at the 390x straight away but then quickly realised its a power hog and would mean that I need to upgrade my PSU. So although the 390x is way cheaper adding a 60-70 pound PSU onto the price means its identical if not more expensive than a GTX 980. I have looked at a load of benchmarks etc on what will give the best FPS in games and it seems pretty close (although 980 seems a bit better) so I have a few other questions.

What card is most future proof for 1080p 144hz gaming? Does the 8gb VRAM really matter if I will not be going to 1440p or 4k resolutions?

What card does best in SLI/CF if I was to buy a second card?

Does the card actually fit my motherboard (see below)?

When do the next generation of cards come out, is it worth waiting? I don't mind waiting another 3-4 months if it means a much cheaper card or a big boost in performance for the same price. I think my PC is slightly above average but at the time I blew my budget on all the other components and kinda skimped on the GPU so that is my first thing to upgrade.

Here are my current PC specs:
CPU - I7 4770k @4.4ghz
MOBO - Gigabyte Z87-D3HP
RAM - 8GB HyperX 1866mhz
GPU - HD 7950
PSU - Corsair CX 600w 80+ bronze
Storage - 128GB SSD , 2TB HDD
OS - Win10