well the 780i has 8 phase power(750 has 6 phase), more SATA connections(2 more right angled), 2 more USB connections, 2 on board NIC's, TRI-SLi, 533/800/1066/1333Mhz+ FSB speeds compared to only 1066/1333mhz on the 750i.
i dont see why you would spend the extra dough unless you were running Tri-SLi and had a ton of SATA drives (hdd's and optical) and really wanted HDA Azalia audio.
you can get on theEVGA site and compare the 2...