this pcb sag is pretty common and it is not really something to worry about. on the other hand i have a very long tahiti le (it is 0.2" smaller in length than a 660ti) i have just used the power cables to help keep everything inline, not so that it binds the power cables but so that the power cables are not actually pulling down on the card.
others will use something connected to there hdd area to come over and support the card and still others if you have a way to connect will hang a ziptie or something from the top of your case.