Well I don't know much about the GTX 555's. If you are into the gaming seen then Maybe you should go with the GTX 560 Ti's and two gether they run faster then one GTX 580.
The 560's are a good card for gaming. Now you could wait for the New Nvidia Kepler coming out in about one month and look at the 660 Ti's. I don't even know if the 555's are even better than my 550 Ti's.
When you say safe limit on twin cards. Are you talking about OC'ing them? Or are you talking about how much bigger in GPU you can run on your new PC?. With that PSU you have You could run 570's with room to spare.
No heavy gaming, multiple monitors and many windows open at one time. Would like pc to be able to run
a decent amount of FPS in case some of the grandchildren or great grandchildren would enjoy. If 570's
are fine with 875 PSU maybe install these or if not much gain maybe 560's.
Have a nice day1