Are you buying from a certain website? I don't know any websites for singapore lol.
If you plan to overclock with the 7970, you actually might want to consider a 600-650w so you'll have headroom to overclock. Make sure that it is from a reliable manufacturer (Seasonic, Corsair, Antec, XFX)
For the colors, each color basically represents how fast it is. Black is the fastest with Blue coming in second. The difference between the two isn't that big. I would only recommend Blue and Black, as the Green and Red and slower but cheaper.
You'll also need a cpu cooler to overclock, as the stock cpu cooler wouldn't get you very far.