I personally use a single 550 Ti in my computer, I haven't ever dipped below 60 frames in WoW, SCII or Amnesia, the Dark Descent.
While the GPU may be weaker than some others on the market, It's still not the worst, and will work wonders @1600x900
Edit: http://www.geforce.com/Optimize/OPS/...GTX-550-Ti-OPS