I just got a new monitor and my games run a little slow on 1920 by 1080. I want to beef up my gpu power, either by installing a second gtx 550 ti, or getting the 650 ti. Right now I've found a 650 that is cheaper than getting the same 550.
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.
In general, the best advise for users is to get the single strongest GPU you can afford and avoid the technical issues of SLI and CF configs (like micro-stuttering and the need for larger power supplies). Just something to add to the discussion.