From what I am reading one 550 ti graphics card requires 116 w to run, and needs 24 a on the +12v rails., so in Sli that would be 232 w with 48a to drive two cards. The Psu has two 12v power rails +12V1@24A, +12V2@24A.
You may have to go a bit higher on the Psu because you will also need more amps for the rest of the system. It will be fine with one 550 ti card in the system, but as soon as you add the extra card in you may be in trouble. so I would go higher than a 620w power supply making sure there are more than 48a on the 12v power rails.
116 watts may or may not be the card's TDP, didn't check, but that's just under 10 amps
24a would be the reccomendation for a SYSTEM with the card, don't have to double the amps reccomendation to add a second card