In most cases, the i5-2500k is a better buy. It doesn't have hyperthreading, has 2MB less L3 cache, and stock speed is 100Mhz slower. But it's $100 cheaper and if you're primarily gaming, you probably won't notice a difference.
4x2GB of RAM doesn't make sense. Get 2x4GB instead.
As far as playing all new games on highest settings with a single 6870, no. But you should get high settings on many new games (depending on your monitor resolution). If that motherboard has 2 PCI x16 slots running at x8, you can use the $100 you saved on your cpu to go towards another 6870 for crossfire. Then you'd be able to play most games at highest settings.