I haven't got either card, but in all probability the fps difference will be significant, and in fps games, extremely significant... 76xx cards are mid range, 78xx/79xx are performance and totally outstrip the 76xx.
From experience, early last year I upgraded from a 6600GT to a 6800GT (and overclocked past 6800Ultra). The performance and image quality gains where massive. The performance gains between the 76xx and 78xx/79xx would be similar to my experience.