you have an 350 GHz PSU?
Just kidding you probably mean wattage (I hope). We need more info in the PSU than that though so if you could supply model and brand name we can make sure you get a card that it can handle.
The GTX550 Ti and the HD7750 perform similar. But I'd get this: http://www.newegg.com/Product/Product.aspx?Item=N82E16814127664
Low power consumption, and faster than both you listed for a few dollars more.