Motherboard / CPU advice?

I'd say give a bit more for a MOBO and get a good gigabyte or asus with x87 chipset. They cost about 150-170 euro in my country.
For CPU get an i5 4670 which is around 200 euro.
As for RAM, I'd suggest a pair of 2x4GB sticks clocked at 1600Mhz with low latency (CL 7-8).
Hope I helped :) Any questions are welcome.

Okay, so, rather than shoot for 100 + 300 for MoBo & CPU separately, I'm just aiming for $400 combined, because honestly, a better MoBo would be a better choice.
So, my advice:
MSI Z87-GD45 MoBo. Good, mid-range MoBo for the Haswell line, with a good SoundBlaster audio chipset, and a fantastic network chipset by KILLER.
Then for CPU, an i5-4670k.

Between the two, you can basically do anything you want with the PC. It'll stomp gaming, do some great audio, etc. It'll handle anything you throw at it, core system-wise. Still obviously need a good GPU for gaming and high end video rendering, etc. The total for this is $385 US, which works out to a little less than 300 Euros, but I'm not sure what the taxes and such will be on your end. I'm assuming it's still going to end up coming in under 400 Euros total, even if you have to pay things like VAT etc.

As for RAM, that CPU & MoBo combo will run basically anything. If you want to avoid having to actually go in and custom clock your RAM though, you're going to want to go with either 1600 or 1866. Anything above 1866 will be supported on the MoBo only by going in and setting the MoBo to OC'd RAM settings. If you're comfortable with that, then you can run 2133. However, it's worth noting that increased RAM clock tiers generally come with slower Latency as well, so if you want to actually run faster overall RAM, you'll have to pay a pretty penny to buy 2133 RAM that's at a low enough latency to actually end up faster overall than the 1866. One of the misnomers a ton of people don't really understand about their RAM. So I'd realistically suggest you go with 1866, as that's going to generally default to something like 9-9-9-36 ish, which is liable to be faster overall than most 2133 in cycles. Brand wise, I've had great luck with a few brands (Kingston, G.Skill, Crucial, etc) but am fairly partial to Mushkin RAM, because the Mushkin is usually reasonably priced, and carries a lifetime warranty. I'm a sucker for a good warranty.
Amount of RAM, I'd suggest that 8GB is basically standard anymore, so that's your minimum. If you can, and want to be good for a while without having to upgrade it later, I'd suggest you go with 16GB.

I'd suggest a AMD FX.

Well i have MSI Z87-G41 MoBo and i5-4670k CPU and dont have any problems, so as long as you are not into overclocking, i think you should be just fine with a cheaper MoBo. The price for these 2 should be just a bit over 300 Euros(at least thats what i paid for them). The difference up to 400 Euros shoud go into an aftermarket CPU cooler(even if you dont overclock, the stock cooler for 4670k is not quite the best). I got the NIC F4 from Thermaltake, but beware that you will need a larger case for this. As for GPU, I`m aiming for Sapphire Radeon HD 7950, from what reviews i have read so far it seems to me the best value/perfomance GPU.

Intel i5-4670K
Asus Z87-K
RAM

Just a heads up. Unless you are getting the exact same MOBO, you need to reinstall windows or else you will ave all kinds of problems.

Get a z87 motherboard
and an i5 4670k LGA 1150 socket

i7 is just hyperthreading, which is usually better at multitasking and video editing/rendering, workstation stuff.

The K means it can be overclocked. Use an aftermarket cooler if you're going to plan on overclocking your CPU.

For ram it's DDR3. One socket, so all you need to know is DDR3. Unless you want to know the frequency of that ram, but that hardly makes any difference. Just get an 8GB kit.

Core i5 4670(k version if you want to overclock later on) and an Asus Z87. (Or alternatively grab the H87-pro one if you cant afford that one)

Just a little heads up: If you don't want to overclock - and why would you want that anyways,the gain in performance is so significantly little compared to the loss of warranty and life expectation - there is no need to get those expensive mainboards. The capacitors and stuff aren't really that much better on high quality boards from my experience. My advise, get an ASRock. Ever since ASRock and ASUS split up they manage to build better boards while ASUS keeps getting worse.

Also you won't need the K version of the CPU. As mentioned before, i7 costs a bit more but the gains are not really noticable. At least not in a way that would justify the price. The only gains come in other fields but as you have to ask here want components to choose you won't do that kind of stuff anyways. So i5 is perfectly fine. I wouldn't choose and AMD at all. Their TPD is usually higher and while the clocks might seem tempting, it's really just numbers that in the end don't make a noticable difference at this level.

You might not want to use the boxed fan for the i5 because I noticed a few cases where the temps were troublesome. Shouldn't be hard to find a nice one though. You will also want to use low profile RAM, especially if you use a non-boxed fan for your CPU. 1600 is perfectly fine. The gains of more are, as always, neglectable.