You will see a six port connector on the back edge of the gtx 550ti, make sure you have one of your 12Vdc connectors attached to this port via a 4 pin to 6 pin adapter that should have come with your gtx550ti.
I don't believe your power supply has a dedicated PCIe connector (6 pin) so you must adapt a 4 pin PS lead to 6 pin. It looks like this -> http://ep.yimg.com/ca/I/cputopiaonline_2248_24312261.gif
This 6 pin connection is necessary because the motherboard cannot supply the requisite power via the PCIe bus.
Upgrade your power supply as soon as a possible. You don't necessarily need more watts, you need more quality.