Overall the GTX 550 Ti is more powerful than the Radeon HD 6770. However, it also consumes the most power. Before actually buying a video card you should let us know what power supply you have.
UGH, lol i have a REALLY old psu, i was afraid of this, its a http://www.newegg.com/Product/Product.aspx?Item=N82E16817153052
, im only running one hd, dvd rom and a stock cooler, 3 80mm and one 120mm fan, so im not using that much power. But im not sure if the 12v can handle the eva 550 ti , can it?