Don't buy a 6950 as it's an old card. If going used, best value is going to be HD7950/7970/7970Ghz -- the latter of which will beat 750Ti/950/960 pretty badly. Then the next level up would be a used R9 290.
If you are buying new, with an i3, you'd want a 950/960, with an i5/i7 a 290X/390 or a 970, skip the 980 (unless EVGA B-stock) and go straight for a 980Ti.
Just look at the data of how the cards stack up and see what you are willing to spend, if your CPU isn't bottlenecking the level of card you want to afford. Plus, always consider how much extra GPU horsepower you actually need (you can review specific gaming benchmarks for the games you play/interested in playing). For instance, it wouldn't make sense to purchase a 980Ti for Trine 3 but you wouldn't want to play GTA V/The Witcher 3, Star Wars Battlefront with a 750Ti if you can afford a 950/960/used 280X/7970Ghz.
http://www.techpowerup.com/reviews/MSI/GTX_980_Ti_Lightning/23.html