I am going with a pre-built unit because 1) it´s been more than a decade since I built a system and 2) I live in a smaller country in Western Europe where the shipping costs for individual items would add up quickly.
So, I decided to go with a system from http://ankermann-edv.de/en/. Here is the unit I selected, then the changes I made and briefly my reasons. Could you please comment on whether these changes a) would not affect gaming performance much or maybe even improve the original system and b) do not create conflicts.
Unit name: GTX 780 Phantom Z i7 4790K
Chassis: NZXT Phantom 410 Black SideWindow (CA-PH410-B1)
CPU: was Intel Core i7-4790K 4x 4.00GHz, boxed (BX80646I74790K), I changed it to Intel Core i5-4670K 4x 3.40GHz, boxed (BX80646I54670K) because a) I understand for any gaming there is no need to go above the 4670K and b) saves me 80EUR
Motherboard: was MSI Z97 GAMING 3 (Sound7.1, GLAN, USB 3.0, SATA3, M.2), I changed it to ASUS Z97-K (Sound5.1, GLAN, USB 3.0, SATA3, M.2) because a) I understand ASUS still represents better quality and b) I am not big on sound.
Graphics: kept the Gigabyte GeForce GTX 780 WindForce 3X OC Rev. 2.0 3GB GDDR5, 2x. I decided to keep it, although I would save 100EUR downgrading to Gigabyte GeForce GTX 770 WindForce 3X OC 2GB GDDR5, 2x DVI, HDMI, but I really do want a very good graphics card.
RAM: was 8 GB (1x8) RAM ELIXIR/ G-Skill/ Crucial DDR-3 PC1600 (M2X8G64CB8HB5), I changed it to G.Skill Sniper DIMM Kit 8GB (2x4), DDR3-1600, CL9-9-9-24 (F3-12800CL9D because I understand that Elixir could be risky. I would rather play it safe since it is the same price. Am I losing something by going with the 2 x 4 versus the Elixir 1 x 8?
PSU: was 650W 80+ Bronze NT Cooler Master GX RS-650-ACAA-D3 (RS-650-ACAA-), I decided to upgrade to Corsair RM Series RM750 750W ATX 2.31 (CP-9020055-EU) (semi-pass) for about 40EUR. Is this a worthy upgrade? I understand that Corsair is a better manufacturer of PSUs and that 750W would only be better given the specs of this system.
The cost of the unit comes to about 1,500EUR.
I apologize for the length of the post and I thank you in advance for your thoughts on this system and these changes.
-CPU- If you are only gaming then the i5 4690k is perfect. If you do video editing/workstation stuff then the i7 is a better fit.
-Mobo- I prefer Asus too
-GPU- Gtx 780 is great.
-Ram- 2x4 gb ram is actually better than a single stick since it can run in dual channel which is supposed to get you better performance
-PSU- The RM is definitely a better choice than the Cooler Master PSU but it is still not my favorite. The Corsair RM uses some cheap secondary capacitors but is still many times better than the other PSU option. If they have the option for a Seasonic power supply I would grab that one. Otherwise the RM is ok. For watts 650w is plenty with room for overclocking, 750w is a bit overkill, you would need 850w if you wanted to SLI another gtx 780 in the future.