Your current PSU should be able to run your setup with a single GPU.
I am not familiar with the quality of the Ultra brand so I can not tell you if it would be a good idea or not.
When you upgrade for the SLI, look at a quality PSU from Corsair, PCP&C, Enermax, Seasonic or the Antec Earthwatts line.
If it is unable to power your system, odds are it will randomly shut down under a load.
If you continue to overload a PSU, there is a good chance that it will fail and, in doing so, send a voltage surge to one or another of your components.
The PSU is one of those components you never want to skimp out on.