when keeping that processor that you have in mind I could say that you should get the gtx 970, but there will be a bottleneck if you do that so the gtx 960 is what i would recommend, its also almost half the price and great for 1080p gaming.
your motherboard has 1 pciex16 gen2.0 slot so that is compatible aswell.
but here comes the problem, you have a 300w powersupply and the minimum that nvidia recommends is 400w.
so you could shop around and find a good 500w powersupply but I'm afraid that the cables won't fit in that motherboard ( im not 100% sure i need someone else to confirm this) if this is the case then you need a new motherboard and that means another processor so you will be looking at a new build.
another problem is that your pc case only supports cards up to 9" and the gtx 960 and other similar cards are bigger then that so it will not fit.
the best that i can recommend you is the 750 ti, but that is not going to give you the gaming performance you need.
if you are planning to do a lot of next gen gaming then i suggest you to make an entire new build because there is almost no upgradeability in that pc