Yes, since you are buying a PSU for a single card only, there is no need for an X58 board. Unless you feel like you will have to have a 6-core CPU next year.
i7 860 and a P55 board should be fine if you need a bit more processing power.
CASE: NZXT Lexa Blackline (AU$149 from msy.com.au)
RAM: Was going for the Corsair TR3X6G16000C9. But that is for i7 9xx right? I do prefer Corsair Memory. Any suggestions? 4GB should be enough yea?
Another questions is with the ATI 5870. There are many different priced ones from different companies. Is there any specifics I should be careful of? Or are they similar?