The only things I think are fairly glaring at first are the HDD and the PSU. The PSU is hardly enough wattage for the 570 when all is said and done and gives you minimal OC headroom... the HDD, while not really a major problem at first, does not marry the GPU and CPU like an SSD would.
If you're playing a huge resolution like 2560x1440 for example, a 580 might be a bit better, due to extra VRAM for heavy AA.
Idk why you bought the i7-2600K, if it was just for gaming, then it was a waste, simple as that.
Buying another GPU, sure you could, but you could always just SLI with the 570, since you already have one, but if you really want to go above and beyond the call, just get a 7970, since they make the 580 about meaningless to have now, based on priceerformance.
You could up to 16GB of RAM... especially since RAM is so bloody cheap now. 4GB modules are the way to go nowadays, so idk why you bought 2GB modules in the first place - considering how little money you saved from just going with 2x4GB, unless that was a typo.
Honestly, if I had the setup you have right now, and I had the cash you're talking about having towards a 580/7970, before I would even consider upping to a different graphics card, considering I already have a 570, which is not at all a bad GPU, I would add a dedicated SSD to this rig and a higher quality PSU.
Quick suggestions for good SSD/PSU:
Seasonic X760 watt PSU 80+ Gold (760 watts 80+ Gold will give you great OC headroom and this is also amongst the best power supplies available.)
Crucial m4 128GB SATA III SSD
Kingston HyperX 120GB SATA III SSD
Mushkin Cronos 120GB SATA III SSD
Corsair Force GT 120GB SATA III SSD