I am always in favor of a single great card over sli when a single card will do the job.
The sli gtx560ti will probably benchmark higher, but that is not what we do; we game.
If there is an issue with microstuttering, it will me more annoying than a few less fps.
SLI implies extra costs. A stronger psu, a sli capable motherboard, a more expensive better cooling case, and extra fans.
How good do you need to be for your games? Unless your monitor is 2560 x 1600, or you are looking at triple monitor surround gaming, a single card should do the job.
Some comments on your build:
1) For a gamer, 8gb will be sufficient ram. Don't pay extra for fancy heat spreaders. 1.5v does not need them.
Only if you will use 64 bit enabled apps like photoshop would 16gb be good. On the other hand, ram is so cheap, that I can't argue too much against 16gb.
2) Why not spend some extra on a GTX580? Such a card only needs a good 600-650w psu.
3) What will you use for a hard drive? If you have the budget, get a 60-120gb ssd for the os and some apps.
Get a 1tb drive for storage and overflow.
4) The haf 922 will come with sufficient fans. You can always add fans later.
5) 1000w seems excessive, even for sli GTX560ti. 850w should do it. Also, do not pay extra for modular. You will be using most of the leads anyway, and in a 922, there are plenty of places to store extras. I usw modular only in very small sff cases. I don't know the quality of fractal design psu's. My short list of quality psu's would include Seasonic, PC P&C, Corsair, XFX, and antec.