560 sli will be better, check out http://www.guru3d.com/article/geforce-gtx-560-ti-sli-review/. Might be better with a single 570 and buying a second when it starts to struggle though as I believe the price difference between the 560 to and 570 isn't huge.
If you already have 1 560 ti however a second is the obvious choice, or just hang on til the new 6 series
Do you really need SLI? The money you would be spending getting even 2 x 1gb 560ti's is only just under the price of a single gtx580 or AMD 7950, which would be far superior in terms of future proofing, and really in terms of games out today would max out pretty much anything.
In a years time when the SLI option would start to struggle you would need a brand spanking new card setup, where as with a single 580/7950 a second card to SLI/CF these would be the cheaper option.
What sort of use do you plan on doing with the PC?
OK now i'm interested can you also tell me if a 560ti and 570 sli would be a good option?
No.
As far as I know, with AMD's you can Crossfire some cards with other specific non-identical ones that are similar (I saw a grid of it somewhere), but I'm not sure you can SLI two NVidia GPU's that are not the same model together - though I might be wrong.