you cant run 3 Monitors (aka Surround) on a single Nvidia GPU (Except GTX670, 680) You need a 2nd GTX550 for SLI ! THen it will work. (but you said the GPU only has 2 outputs ? Problem!)
Best way would be to buy an AMD GPU! 7750 or such. they should be able to run Eyefinity aka 3 Monitor gaming on a single card.
Surround on the 200, 400 and 500 series cards works by plugging two monitors into the first card and the third monitor into the second card. So no, no problem. However, the OP will still require either the purchase of a second 550Ti, or another card which would be capable of running Surround from a single GPU (ie: 670/680, as Haldi said).
You won't have to worry about DisplayPort capable monitors on the 670/680 route as long as each monitor is <1920x1200, as I'm running Surround with two monitors via DVI and the third with an HDMI->DVI cable.
You have a few options, but what options are recommended will depend on a) exactly what you want to do (light gaming or heavy?) and how much you would be prepared to spend. ;)