If your budget permits, go for the HD6770. Avoid the GTX550Ti and go for the HD7750 if you cant get the HD6770 in your budget. The 7750 is a good single slot card with performance equaling or better than the GTX550Ti with lower power draw.. By the way, what SMPS/PSU do you have?
As for CPU, go with the Phenom ii X4 970, although if its a purely gaming PC you want, an i3-2100 offers very good performance