It'll be fine. The 580 can handle anything you throw at it.
The only real issue I see with the build is that you're using dual channel RAM on a triple channel board. The RAM will run slower than a similar triple channel kit would. I'd recommend trying to find a single 4 GB stick that is exactly the same as the two you already have. If you can't, see if you can find a good price on the 8 GB kit and buy a similar
3x2 GB kit for $80.
EDIT: I may have found a stick that likely matches the kit you have:
G.Skill Ripjaws 4 GB 1600 mhz CAS Latency 9 1.5V for $55. I'm not positive if that's a match, so check the timings and speed on the sticks you have. I know there was a
1333 mhz CL 9 stick for $41 as well. Assuming the 8 GB kit is 1600 mhz/CL 9 and it cost you about $100, if you sold the 8 GB for more than $55 and bought the 3x2 GB kit I linked above, you'd come out ahead.
There are a couple of changes I would have suggested before you bought the parts.
First, I would have also advised you to avoid Coolermaster PSUs, as they're known to be pretty low quality. Instead, I would have gotten an 850W from Corsair, Silverstone, SeaSonic, Antec or XFX. 850W is more than enough to use dual 580s and overclock, so the extra 150W is pretty much wasted money, both through the added cost of the unit and overall efficiency.
Second, I wouldn't have gotten a LGA1366 system at all, but that's merely timing. The Sandy Bridge CPUs (released in January, but essentially recalled due to a motherboard design issue, soon to be re-released) absolutely destroyed any reason to buy the LGA1366 socket. The i5-2500K ($225) and i7-2600K ($330) both performed close to the i7-980X ($1,000) at stock, and overclock to a ridiculous level. I would have strongly advised waiting for them to be available again as their performance gains were astonishing. Couple that with the fact that the motherboards were about $100 cheaper than similar X58 boards and they used dual channel RAM, and you've got a killer bargain.