Dimitris,
NVIDIA: By far, the best GPU of all for the i7-4790 is the GTX 980 Ti, so the answer as to the best GPU for € 200 would be GTX 980 TI that costs € 200, But, even used, these are still € 500 on ebay.de. On Passmark Performance Test baselines, the GTX 960 produces 3D scores up to 15735, which I take to be a pair in SLI.
It appears that the highest performing new NVIDIA for € 200 is the GTX 960. On Passmark Performance Test baselines, the GTX 960 produces 3D scores up to 7642. The highest scores all seem to use Z97 motherboards by Gigabyte (Z97ZUD5H), MSI Z97 7918), ASUS Z97 Pro Gamer), ASRock Z97 Anniversary) The Passmark average for the GTX 960 of all systems tested is 5954. The problem with Passmark averages though is that they include tests of systems in which the system sores 0.0 or very low.
If you don't mind buying used, with luck you could buy
two GTX 780- Ti for € 200. The highest Passmark 3D score for i7-4960 / GTX 780 Ti is 11899- probably a pair in SLI. As you can see, that is significantly higher than a GTX 960 and better than a single GTX 980 Ti. That would be my choice, as I buy all my graphics cards used but those are Quadros which are less stressed for reliability. Not everyone likes used GPU's but I've had good luck .
AMD: AMD cards often seem to provide higher performance than NVIDIA for the same cost. The very highest 3D scores for i7-4790 / AMD R9 is 10934 from a Radeon R9 Fury X on an ASUS Z97 Deluxe and the i7 is running at 4.4GHz. That card is about € 550.
However, in this case the NVIDIA € 200 card appears to win over the € 200 AMD. The best performing R9 for € 200 appears to be the R9 380, which produces a 3D score of 6977 and the Passmark average is 5858. That is on MSI Z97 MS-7917 and with the i7 at 4.4GHz.
In the used R9 world, it'spossible to buy a used R9 280X for € 200. The top score for the 280X is 9250 - which may be a pair, but scores of 6500 seem quite consistent. My sense in AMD R9 is that the R9 380 was a jump in technology and in this case, the new card is the better choice.
The clear winner in general would be a pair of GTX 780 Ti on a Z97 motherboard.
Cheers.
BambiBoom
1. HP z420 (2015) > Xeon E5-1660 v2 (6-core @ 3.7 / 4.0GHz) > 32GB DDR3 1866 ECC RAM > Quadro K4200 (4GB) > Intel 730 480GB (9SSDSC2BP480G4R5) > Western Digital Black WD1003FZEX 1TB> M-Audio 192 sound card > 600W PSU> > Windows 7 Professional 64-bit > Logitech z2300 speakers > 2X Dell Ultrasharp U2715H (2560 X 1440)>
[ Passmark Rating = 5064 > CPU= 13989 / 2D= 819 / 3D= 4596 / Mem= 2772 / Disk= 4555] [Cinebench R15 > CPU = 1014 OpenGL= 126.59 FPS] 7.8.15
2. Dell Precision T5500 (2011) (Revised) > 2X Xeon X5680 (6 -core @ 3.33 / 3.6GHz), 48GB DDR3 1333 ECC Reg. > Quadro K2200 (4GB ) > PERC H310 / Samsung 840 250GB / WD RE4 Enterprise 1TB > M-Audio 192 sound card > 875W PSU > Windows 7 Professional 64> HP 2711x (27", 1920 X 1080)
[ Passmark system rating = 3844 / CPU = 15047 / 2D= 662 / 3D= 3500 / Mem= 1785 / Disk= 2649] (12.30.15)