The GTX 570 is [slightly] faster than the GTX 560 Ti 448 Core Edition.
The 448 will be able to OC to a level of which the 570 performs. However, if the 570 is OCed, then it can perform just as well as a 580 (stock speed). I would say, buy the 448 if you are willing to change graphics card within the next 2 years, or the 570 if you are willing to change within the next 3 years.
Cheers, just wanted to be completely sure before ordering