GTX 550 Ti's in SLI are roughly just under a GTX 570, depending on the game of course. You can only SLI a GTX 550 Ti with another GTX 550 Ti.http://www.xbitlabs.com/articles/graphics/display/geforce-gtx-550-ti-sli_9.html#sect0
The current generation single card that is closest to a GTX 570 is the GTX 650 Ti Boost or GTX 660.http://www.techpowerup.com/reviews/XFX/HD_7790_Black_Edition_OC/26.html
Since a new GTX 550 Ti costs $105-$120, I would recommend going with a single GTX 650 Ti Boost for $150 (1GB) or $170 (2GB) and selling your current card. Better yet, you could go for the GTX 660 2GB for $190 and get a free copy of Metro: Last Light.