The 7750 pulls 75 watts max TPD. The 660 Ti pulls 150 watts max TDP.
On a 400 watt PSU go no higher than 115 watts. That limits you to a 550 Ti or lower, or a 7770 or lower.
I have a Corsair VS 450, can i go with HD 7770 1GB. I want to play games like Batman, AC, Far Cry etc.