Big question about pci3.0

alot of people are saying that there is not much of performance difference between pci 2.0 n pci 3.0 when having a ati 7870 graphics card. people are saying its a waste of money to go for z77 mobo if u r buying a i5 3450 cpu

so what exactly does this pci 3.0 does?
will there be a increase in gaming performance if i install a z77,i5 3450 n 7870
or will it be the same as p67,i5 2400 n 7870

also suggest best mobo for i5 2400 n i5 3450 with a budget of 22k for both cpu n mobo

thanks in advance