The difference between the i5-2500K and i7-2600K is the Hyper Threading; HT + Gaming = little; rendering or encoding the i7-2600K is the better choice http://www.guru3d.com/article/core-i5-2500k-and-core-i7-2600k-review/20
I prefer ASUS, so either the P8P67 PRO or P8P67 DELUXE. The MSI P67A-GD65 is good as well.
As far as i7 9xx vs i5/i7-2X00K, it depends on the resolutions. I'm not a big fan of pairing-up GTX 580 in SLI to a Sandy Bridge. A single GTX 580 is fine on a P67.