From what I can tell the SLi-550Ti uses more power
and less performance
than a GTX570.
That being said the differences weren't enormous.
I'd personally get a single GTX570 just so that in the future I could add another one for a greater boost in performance.
Other 570s suggest a -minimum- 550w PSU with 38A on the 12v rail.
Please read through the thread thoroughly before responding.
Nobody wants to read a wall of text, formatting is your friend.
i5 3570k | 8GB G.Skill 1600MHz | R9 290 | SeaSonic 750X