Hello and welcome to the forums
This topic will probably help you about SLI/CF http://www.tomshardware.com/forum/245454-33-crossfire-f...
Anyway,2 560Ti's will give you the performance of 1 580(even faster in some cases) but they offer more noise,heat and power consumption.I personally prefer a single 580 but if you can't afford it,then get another 560
A 560Ti goes for ~$200. so the best thing you could do for the same cost is crossfire 6770's or SLI GTS 450's. Two cards might be faster but you'll have no future upgrade path, and you'll also have extra heat, power consumption, and noise. Also, you might experience micro-stuttering, which could be annoying.
I'd go with a single card before going with a double card. Buy whatever single card plays the games you want at the settings you want. A few years later when you aren't getting the performance you want you can add a second card cheaper than buying a newer single card.
What is your budget for getting a video card and if you spent the budget on a single card how long would you have to save to get another. This is just a way to see what is the best card or cards setup you can get in a resonable amount of time.