I am not sure whether they will be able to overtake South Africa in NRR. At this point, Proteas have +2.290, while India is at +1.405. Even if India win by 300+ runs, there is a chance that their NRR may stay below that of South Africa. Anyway, India will be having 7 wins from 7 matches, while South Africa lost the match against Netherlands. And next match for both the teams is going to be at Eden Gardens, where they will face each other. If anyone is going to beat India in this world cup, it is going to be South Africa. This match is going to be epic.
India couldn't beat the NRR of South Africa despite winning the match by 300+ runs. India is quite impressive in this world cup, both batsmen and bowlers are in form now. I also think that the winning streak of India might end against South Africa in the group stage. South Africa has scored above 300+ runs in all those matches when they batted first, and won those matches by a huge margin. SA has batted second in two matches, they lost one of them and narrowly got the victory in the other match. So, I think India won't allow South Africa to bat first if they win the toss.