Your PSU should be fine if and when you decide to go with 2x5870s as long has your not overclocking anything. The PSU calculators I looked at put the consumption between 550-600w for your system with 2x5870s and no overclocking.
But I can find no fault with the MSI you chose. Just a preference for Gigabyte boards.
I'd also go with a different case, I believe in spending for a top quality case since it's likely the one component of a PC that will last you forever. But this is just my philosophy and what makes a good case is often subjective.