Are you getting an error or something? My Core 2 Duo MacBook Pro from 2009 runs iMovie just fine.
The fact that the cards are both 700 series means nothing. The 750ti is more similar to the 900 series because they are both built on the Maxwell architecture. The 770 was released 1.5yrs before the 750 and is based on the Kepler architecture. They are very different cards.