I have great news!
I took the celeron out and put the 2600K in.
After post -> all LEDs = out!
After 1,5 hour of torture in prime95:
Conclusion:
- 2600K not broken
- motherboard not broken
- GD2 led is just an oranje led that goes either on or off. After the post, before windows-startup, the GD2-LED goes out together with the rest.
Decreasing voltages in the bios does not make the LED-status change, this happens AFTER the post.
- Spontaneous reboots were no instability-signs. Some gigabyte motherboards do this after a bios flash or after changing a lot of settings and saving it to the cmos.
- I think the stripes and artefacts in-screen and the not-wanting-to-post-to-bios issues was really a bios-F1-issue. I really don't know what else it could be, I excluded everything now. On this site I can see that it is supported since F2:
http://www.gigabyte.co.nl/support-downloads/cpu-support-popup.aspx?pid=3909And no, I am not willing to flash back to bios F1 again
The torture....
So for now, the quick conclusion is that my system is just fine now.