Here's a quick linkhttp://www.videocardbenchmark.net/gpu_list.php
I assume you mean the GTX 550 Ti
They're both essentially equally matched, but the GTX 550 Ti does come out slightly higher and I'm an Intel + nVidia fan (after having tasted AMD and ATI).
So as far as graphics cards go, I'd recommend the 550 Ti.
I've heard good things about the Phenom II X2, but your mileage may vary.