the hd 7870 is probably too powerful for 720p but if u can afford it, it wont be bad.
i mean: now it is an overkill but after 2-3 years it wont be, and if it is....
then after 4 years it DEFINATELY wont be.
and if u decide to buy an HD monitor in the future u wont have to buy a new graphics card.
it's your choice, if u can afford it go for the 7870.
Where would the 660 fit in there?