Well Dude
Right now Screen Res is 2560 X 1600 => which is going to 4k in near future.
Right now I have a craze about diablo 3. But I have known to play car racing games too, FPS games. I bought my 7970 when it was highest tier and paid $300s to NCIX at local.
Right now Built in 4290 HD is doing all the jobs except Video play and Games.
SO I can wait for little while.
Can you please specify your actual CPU and its clock speeds? This is a big deal because as far as I am aware, HD4290 was a built-in GPU on the completely outdated AMD 890GX (790GX) chipset. If so, you will need a entire CPU platform upgrade.
http://www.anandtech.com/show/2952/2
Assuming you do
not have an outdated Phenom II/FX6000-9000 series, then:
--------------------------------------
For Diablo 3, you do not need anything faster than an HD7970 for 1440p/1600p.
For 4K, for Diablo 3, R9 290X/970 and therefore RX 480 will be more than enough as well.
You have to get really specific about what racing and FPS games you intend to play at 4K.
Generally speaking, GTX1070 is not fast enough for 4K for modern AAA games no matter what some people on here try to push. For instance, if you play a lot of Project CARS and intend to game on 4K, you'll really want at least a
GTX1070 OC for that title.
For Need for Speed on highest settings, GTX1070 SLI no less.
1070 clearly not fast enough for 4K beyond low/medium settings. At High/ultra it falls apart completely.
https://www.youtube.com/watch?v=BHH-ycqpYuA
1080 isn't even fast enough to get 60 fps averages.
https://www.youtube.com/watch?v=mrRhV6qfrr0
If you actually intend to keep these cards for 4K to play the latest AAA games over the next for 2-3 years,
you really would need GTX1070/1080 SLI. Of course all of this assumes you have a CPU at least i7 3770K @ 4.5Ghz class level or i7 2600K @ 4.8Ghz. Another alternative is you get a GTX1070 card that lets you play the most demanding games at 1080p 60Hz and if you have enough GPU horsepower, you could use 4K on the 4K monitor. Of course all of this assumes you have a modern solid CPU.
Otherwise, 1440P 60-144Hz is the sweet-spot for GTX1070/1080 cards at the moment.
I think a 400$ 1070 is your best option.
Not if the OP has an outdated old generation i3/FX series CPU it isn't.