feelinfroggy777 :
(edit) and gaming is fairly popular, and I can assume the OP will be doing some sort of gaming, which means that CPU won't be that good at 2.8ghz and won't overclock, that CPU would get crushed by even a much older i5 or i7, even though it's 6 cores gaming can't really use more than 4 cores anyways, so unless the op wants to do gaming and streaming or something that can take advantage of higher core counts, that CPU would be crushed, even in streaming.
Not sure what your talking about here, but the 8400 is a great gaming CPU and will do just as good, if not better than a 4790k. His only real upgrade option is a 4790k and I would only buy one for a good price because the OP can move to a new platform for 400 euro.
So unless you can get a great deal on a used 4790k for less than half of his budget, then I would go with the 8400 and platform upgrade.
You do realize your trying to get the OP to upgrade to a 2.8ghz CPU, gaming can't take advantage of all 6 cores unless your playing CPU intensive games like GTA 5, in which case it still wouldn't do too good because the CPU still needs to be a reasonable speed, and 2.8ghz just isn't fast enough for gaming these days, even on a 6 core CPU. Yeah it will game, sure, but it may not be much faster for gaming v.s what the OP could get on his socket, and older gen CPU's actually aren't that much slower than modern Intel CPU's, the 8th gen excluded since the upgrade to 6 core CPU's. The point is the op will see a bigger increase with 16gb's of memory V.S 8, a much higher speed Quad core V.s a slow speed 6 core, and a slightly slower speed Sata SSD. I don't see the point in buying a completely new CPU, motherboard, memory, and an M.2 SSD when it won't be that much faster, I would guess around 10-15 FPS faster than the OP's current system in most games, depending on the GPU. Also, we still don't know what the OP is planning to do with this system, if the OP is planning to game, I would push the OP away from that system as I find the CPU would be too slow per core for most modern games, if the OP wan'ts to do things like content creation or CPU core count heavy tasks then I would recommend this system.