I'd say a single 560Ti as it will certainly be less problematic. While it's true that in general two 550Tis should be faster, your CPU is a bit aged by now. In fact, if the CPU is the limiting factor it might be slightly slower with the two 550s simply due to the CPU overhead of dividing up the work.
I would say don't do the GTX 550 Ti's if you want do all the top games. The 550's are good for strategy, puzzle, some arcade type, and some RPG. How ever they do struggle on the more GPU intensive game.
The reason I know this is I am a owner of two. They are not worth the money. You would be better off with one AMD 6870 or even two than the 550's. I hope this helps some and good luck to ya