2x 560ti hawks, like I do! Basically a hawk performs like a stock gtx 570, so you can think of having sli 570's in your system. Just make sure to overclock each one to 1ghz and you should easily surpass the 580.
Single GPU set ups are always a lot easier to manage than SLI with heat, power and user friendliness - that would be the only reason I'd buy a single GTX 580. However, SLI is very good and I always say two is better than one. I have two GTX 580s in SLI myself
It's easy to install, I recommend uninstalling your graphics card drivers before adding the second card into the machine though. Boot up again and there should be two display adapters showing in your device manager. Install the latest drivers and there will be a notification telling you that your system is SLI capable and gives you the option to enable, you click Yes and BOOM you're set.
SLI takes the load on your graphics card and splits it between two graphics processing units, which allows both of them to work harder together. So any effects such as AA are executed much more efficiently and you will notice a drastic improvement in your SLI supported titles.