Need suggestions: Gaming PC +/- 1500 euro

Hello there!
I decided to build a new PC, mainly for gaming purposes. I chose a maximum budget of 1550 euro. I will mainly be gaming on a 50hz 1920x1080 resolution screen with 100hz. The main idea is to overclock my CPU and GPU and be able to play games on highest settings for the coming few years and later. I'm however new to this and would highly appreciate some input from more experienced people like you. Sadly, I'm restricted to parts from Europe (I'm from Belgium) because of expensive taxes on components shipped from the US.

Here is the build (~1500 euro):
Case: Corsair Carbide Series 500R (95 euro)
PSU: Corsair RM850 (130 euro)
Motherboard: Asus Maximus VI Hero (173 euro)
CPU: Intel core i5-4670k (190 euro)
CPU cooler: Noctua NH-D14 (70 euro)
GPU: EVGA GeForce GTX 780 Dual Classified /w ACX Cooling (530 euro)
Memory: Crucial Ballistix Sport 2x8GB 1600hz CL9 (114 euro)
SSD: Samsung evo 840 250gb SATA (120 euro)
Drive: Asus Blu-ray writer 90DD01K0-B30000 (61 euro)

I won't need a monitor, mouse, keyboard, large HDD and OS. I tried to get the cheapest prices by buying from multiple webshops through pricewatch. I took the Asus Maximus VI Hero mobo for the OC possabilities, sound quality and second PCI-e socket in case I'll get an extra GTX780 for SLI later. I'm planning to OC the CPU with the Noctua NH-D14 and the GTX780 with the built-in ACX cooler. I'm not planning to use water cooling as I'm new to this.

Few extra questions:
Is the EVGA GTX780 Classified /w ACX worth it over the cheaper Asus GTX780directcu ii oc and MSI GTX780 gaming?
Should I aim for an i7-4770k for the hyperthreading possabilities instead of the i5-4670k?
I noticed that some people dislike the RM850 PSU because of the cheap capacitors, would a seasonic m12ii bronze 850w be a better choice?
Should I keep the 2x8gb RAM and keep 2 slots open for future use or fill all slots with faster 4gb sticks?
Would the airflow in the 500R be sufficient with the stock fans?

Any input is highly appreciated,

thank you very much!

I'd prob grab the MSI gaming 780.
i5 4670k is plenty for a gaming rig.
If you're not looking to upgrade to SLI later on, grab a Seasonic G 550/650 and be done with it.
Keep the 2x8GB
500R has plenty of airflow. Absolutely plenty.

I reckon you could save a few bucks in some places. The Asus Z87-A would be fine as well. The Corsair 230T would be another case to consider? (Or 200R to save a few bucks).

The MSI gaming 780 gtx would save me about 95 euro to spend at other components, will it perform well compared to the evga?
Alright, I'll stick with the 4670k.
I was planning to upgrade to SLI later, I could maybe get a cheaper mobo such as the Z78-45 gaming and get a MSI gaming 780 gtx ti instead of paying for the extra PCI-e socket.
The 200R indeed seems to be a better choice, as I won't need all the HDD space the 500R offers. Between the 230T and 200R, which do you think is the better option?
I mainly went with the Maximus vi hero for OC capabilities and better sound quality.
I'd prefer to stick as close to the 1500 budget as possible, while getting the most out of it.

What are the AMD prices like where you will be ordering from? 290 or 290x might be worth looking into.

I've always had grand plans when I buy a nice video card to add a second one down the line, when they get cheap and "double" my performance for cheaper than getting the "new best card". It's honestly never worked out like that. I'm always able to find a great deal on the "new best card" and I'm able to sell the card I have for about the same price. I'm not "doubling" my performance, but I usually get about a 40-50% bump for very little money out of pocket.

If you don't plan to SLI, you can definitely save money in the PSU. If you can, I'd go Seasonic. Maybe 550-650w.

For the motherboard, you won't be OCing any higher than 4.5, even with a nice air cooler, so there's no need to drop a huge chunk of money on a nice board. Z87-a is fine, as mentioned. Extreme 4 would also work.

http://www.videokaartshop.be/category/207601/amd-radeon-hd-videokaarten.html?10782=53305,53304

This is the cheapest store here for video cards,
some 290/290x pricings:
Gigabyte GV-R929XOC-4GD - 540 euro
XFX Radeon R9 290 Double Dissipation Black Edition - 440 euro
MSI Radeon R9 290 Gaming 4G - 400 euro
MSI Radeon R9 290X Gaming 4G - 510 euro

I think I'll stick with the Z87-a instead of the hero vi as I'm not going for extreme overclocking. Would it be better to get the the Deepcool Neptwin instead of the Noctua N-DH14 as it seems to perform almost as good for 25 euro less.

I guess by the time I'd consider SLI, I might as well just build a new PC because other parts such as the CPU needs an upgrade.
I'd prefer to keep a 850W PSU though as I'd like to reuse it in future builds if possible and I guess they will consume more energy anyway.

The 290X is going to beat the 780 in almost all situations, so it's definitely the better buy at those prices. 290 will even compete with the 780 and it'll save you quite a bit.

Unless you're going to do dual cards, you don't need 850w. Even if you use it for your next build, power requirements won't go up that much (if at all). You could at least bump down to a 750w unit and save yourself a bit of money (and still probably be able to handle dual cards).

By saving on the board. Dropping to a 650w psu (if it makes a difference I was running an i7 4770k with a gtx 670 on an sfx 450w psu. I never broke 300w.)

Grab yourself a 780 Ti. And be done with it. If you're at 1080p it's way more than you need and will last you a good few years. If you're at 1440p it's a great card for that res.

Corsair 230T comes with an extra fan and led front fans. The 200R by default has two fans. No led. There is a windowed model though.