Generally speaking, the lower performance the card has and the less power it draws, the less heat it will generate. Therefore, a Radeon HD 6450 produces less heat than a Radeon HD 6970, but due to the smaller heatsink on a Radeon HD 6450, it's operating temperature will not be much lower than the Radeon HD 6970 with it's massive heatsink in comparison.
The Radeon HD 7750 is an example of a card capable of providing great performance based on it's low maximum power consumption of about 43w. It's performance however, does not compare to the GTX 670 which has a max power consumption of 162w.
You should probably take a look at the following GT 670 review which includes power consumption, performance per watt and performance per $ charts to help you make a decision.http://www.techpowerup.com/reviews/NVIDIA/GeForce_GTX_670/26.html