The 650 ti doesn't have as much performance as you might think, so two 550's would probably perform better, BUT the 550's will be limited by their 1GB of VRAM in games like Battlefield, Crysis, etc. I would try selling your current 550 to get maybe a 660 or 660 ti for a ton more performance, or just wit until the next generation of GPU's comes out in a few months, your call.