900 is huge overkill, a quality 600W psu would allow you to overclock your current setup easily. Corsair makes one of the best psu around so you are good there. If you were going to sli, you would be pushing but you would be okay. http://www.bjorn3d.com/read.php?cID=2006&pageID=10203
I would personally go with the HX 750, it is modular and high quality.