as was said, the 560 448 is a defective 570. Nvidia stockpiled the broken ones for several months for the special holiday release of them. so if you think you would EVER want to SLI don't get one. there's no guarantee Nvidia will keep releasing them or there will be enough defective 570s to maintain the supply.
It is actually listed on the Nvidia site as a 'Limited Edition', so if you were to SLi them then get two at same time and sit out the next couple of releases.http://www.geforce.com/Hardware/GPUs/geforce-gtx-560ti/specifications