Generally speaking GeForce cards use more power than AMD cards. On the low end it be about 25% more power.
The best card will probably be the HD 5750. Gaming performance is roughly between the GTS 250 and GTX 260 (closer to the GTX 260). All those cards are more powerful than the 9600GT. The HD 5750 consumes about the same amount of power than the 9600 GT (72w) while the nVidia cards consume more (GTS 250 is 113w, GTX 260 is 109w) under peak gaming conditions. That means the nVidia cards uses about 50% more power.
See following review which includes a "regular size" HD 5750 and a low profile HD 5750 (probably more fan noise due to small fans spinning fast).
http://www.techpowerup.com/reviews/Powercolor/HD_5750_Low_Profile/6.html
The lowest priced HD 5750 is selling for $100 after a $10 rebate:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814161309&cm_re=hd_5750-_-14-161-309-_-Product