Is i3 4130 pairing with a GTX 750ti good enough for casual gaming on 1080p medium settings with decent fps (30+), for games like Diablo III, War Thunder, Civ V etc?
I'm on a budget and wondering if I should just wait for a few months to save up for a i5 4670k and gtx 760 combo.
That combination should be fine for 1080p on medium. I was in your position a little while ago and I decided to wait and get the same thing you suggested (i5-4670k and GTX 760) for the extra performance and the ability to play on higher settings and for the overclocking potential of the 4670k. The choice is yours though, because the GTX 750ti and the i3 will be fine for what you want.
It doesn't take much to max out Diablo 3. You actually should be able to with the 750ti. Not sure about civ and war thunder though.