The only thing that i am worried about is games that don't support SLI then you might not be able to max it out with the GTX 560 ti whereas with the GTX 680 it will run the game on a single card decently!
the gtx 680 is one of the sigle powerful card it will be better to get one powerful single gpu between the two gtx 570 3 way sli and gtx 680 2 way sli in my opinion the gtx 680 2 way sli will be the better choice.
If i was going to buy a new graphic card setup and was ok with spending around $1200, i would wait for the GTX 690 to be in stock. Its being released today (May 3rd). Might not see them in stock right away tho. GTX 690 is just a pair of 680s on 1 card. A dual GPU card.
The 690 will preform much like 2x 680s, but require less power and have less noise. a 680 is said to use 195W and a 690 300W. So a good 750W PSU will be able to power your system. nvidia says 650W PSU is required for a 690, best to have some head room. A 560 ti will use 170W per card. Note I'm using TDP from nvidia's website so the power usage may vary.
If you don't want to wait then 2x 680s would be my next choice. Newegg doesn't have any GTX680s in stock ATM. When evga 680s are priced at $499 i really wouldn't spend $650 for a 680, I'd wait until they are in stock. Buying factory OC'd cards are a waste of money IMO. You may get lucky with the release of the 690 here soon and grab one the first wave of stock. The MRSP is $999 which is on target with being the same price as two 680's.
The power requirements will be lower then a tri setup, the space needed in a case is less with only 2 cards, more mobo options for 2 card setup, less heat with 2 cards. less noise vs 3 cards, less hassle overall with 2 cards vs 3. (same with 1 690 vs 2x 680s)
To be honest a tri and quad SLI setup are for barging rights.
Another thing most people overlook is that Vram with SLI and crossfire aren't combined for every card you have. So a Tri system doesn't mean you have 3GBs of VRAM. You only have whatever a single card has, so in this case 1GB. Why? Because how SLI works is 1 card renders a frame while the others render another. They aren't working together on a frame, they are just alternating rendering individual frames. example GPU1 - Frame 1, GPU2 - Frame 2, GPU 1 - Frame 3, GPU2 - Frame 4...etc. So a 680 or 690 with 2gbs of Vram has more Vram that is used to render each frame then a Tri SLI 560 ti's(1GB of VRAM).
the gtx 570 3 way sli will do better but will not a good one to get the gtx 570 3 way sli will beat 2 way sli gtx 580 which is about 5-10% slower then gtx 680 or lesser or more.http://www.demama.****/hjk