gaming rig?

Not bad, but buy another PSU. 500W is more than enough!

And a Dual graphic card? Mhmm, well buy a different one. Maybe a GeForce GTX 580 is better than a dual graphic card.

A dual 6870 setup will outperform a single 580 gtx, and at a lower price.  500W is not enough for a dual GPU setup.

With only one hard drive I would consider finding a 10 000 rpm drive instead. IMHO bluray on a computer is a waste, unless you're going to use it as your media station.

One great video cards will be better than two lesser ones in someways, particularly on that MoBo as the PCIe connections for them will be running at half speed. And you will only have the memory of one card. Also consider that some games do not play well with dual GPUs. One card would be cheaper and by the time the extra power is worth having you could upgrade to a new GPU that will likely be more powerful than both of them together (and by then 1 GB of video memory may no longer cut it).

I would also recommend the LIAN LI Lancool PC-K63 Black Steel ATX Mid Tower Computer Case. They are fairly equal, but this one is about $50 less.

Might be worth getting a small SSD to install the OS on, such as the Crucial M4 CT064M4SSD2. Also two Wester Digital Caviar Black 7200 RPM 500GB in Raid (I think 0 is striping) would be better as smaller drives tend to be more reliable.

If you're going to buy AMD, get an ATI video card.  Not so much for the performance, but because AMD verifies that compatibility platform a lot better because they're the same company.