So im just guessing you are limited to 300-350$
There is also another option partially covered here about CPU: http://www.tomshardware.co.uk/answers/id-2762297/upgrade-920.html#16449730)
Basically if you mobo is fairly good enough you could try putting: Core i7-9xx, Core i7-9xxX, Xeon 36xx, Xeon 56xx.... especially pointing to the 56xx 6 core Xeons. They can be bought second hand below $100. there is 200-250$ left so thats a range of GTX 960, 280/280x, 380... so the middle of the stake between 7870 and 970.