Your Antec TP3-550 is a five year old psu (power supply unit) that is made by Seasonic.
The general recommendation is that you should not run your psu higher than 80% of its rated power output.
As long as your system doesn't demand more than 440 watts for extended periods, you should be OK.
It's hard to say how much longer your psu will work, but Seasonic is one of the best psu manufacturers.
Adding a second video card is not recommended with the TP3-550, especially if you overclock your cpu and video cards.
A high quality 650 - 750 watt psu is needed for an overclocked SLI system.
My Antec TP-750 New psu is also made by Seasonic and works great with my overclocked E6850 + N650SLI-DS4 system with two GTX 460 video cards.