Here you can see most of cards benchmarked on cryptonight algo (data may be outdated after the fork - but shouldnt have huge difference)

http://monerobenchmarks.info/

From my experience, there shouldn't really be any kind of extra ordinary difference b/w gens of graphics card as far as mining is concerned. You can check older cards and see for yourself, one major aspect is the power draw - which sometimes can get lower or higher (depending on specs), but I dont think it is much of an issue when considering the total performance h/s/w wise, also taking into consideration current price/difficutly etc..

If you are getting into mining game, make sure you are pretty sure and confident that it will give u the calculated ROI.