Well if u double the one u have u should be set but should u start doing heavier rendering its always good to have room for improvement, get the 2X8gb kit if its not too expensive, if it is just go with doubling the one u have mate
A good 600W PSU would be sufficient (i.e. not the Corsair CX) for a single card. The TX and HX lines are better if you want Corsair, but I'd go for Seasonic or something they build, like all the XFX and many of the Antecs. The Delta-built Antec units are great too.