The 7870 XT is the AMD equivalent of the 560-448. The 560-448 was a 570 with a broken SM. the 7870 XT is a 7950 that has been brought down" to 7870 level. I'd get the 7950 if for no other reason that it's not an oddball and wuld be easy to match or replace.
If you don't want to absolutely max games (i.e. turn every setting up without even bothering to see what they do) on your resolution, a 7870 XT will be MORE than enough, and will let you get a nice mouse. Personally, I like my peripherals and consider it by far the better option. (Though I did spend $120 to import a keyboard from England, so...)
That being said, Jack Naylor has a good point in that the card will be hard to find later down the road if you want crossfire in the future. However, he undersells it a bit; it's a great card.