Wow that is so freaking helpful Thank you very much! The PSU indeed is 220-G2-0750-XR model
I will mostlikely get the 8gb now and then try to add 4gb more if needed in the future, hopefully it will work.
I've chosen black due to its 5 year warranty, really low failure rate and fast overall and tbh I dont want to change it much
Im not so sure about the 780 though (compared to 290)... pros: lower heat, lower noise, good drivers, low failure rate, little bit lower power draw (not that it matter much), shadowplay, no blackscreens as on 290/290x as i've heard... and there is quite lot of them
but on the other hand the con: 40% more price for 15-20% performance gain soooo thats quite tough decision... although i will save quite bit on those rams so it shouldnt be a problem.. but still Any ideas?