@ TS,
your CPU is still very sweet... palit ug nice na aftermarket cooler then i-OC na... that i5-2500K is one of an OC'ing beast... go go go...
for your GPU, pili lang according to your budget... kun dako2 ang budget, go GTX 970 but be sure, you'll only use that in a 1080p resolution para dili mayabag ang fps... if limited gaman ang budget, the GTX 960 will do the job nicely... pero, kun maka huwat ka gamay, coming naman ang GTX 960 Ti... or the equivalent offerings from AMD... kaso, no idea when kaha ni mahitabo pud...
for now, the best GPUs would be, the GTX 960 and 970 @ 1080p gaming... the "Power consumption to Performance Ratio" is the basis of this statement of mine...