For a system using two Radeon HD 6870 graphics cards in 2-way CrossFireX mode AMD specifies a minimum of an 600 Watt or greater power supply. The power supply should also have a combined +12 Volt continuous current rating of 45 Amps or greater and have at least four 6-pin PCI Express supplementary power connectors.
Total Power Supply Wattage is NOT the crucial factor in power supply selection!!! Total Continuous Amperage Available on the +12V Rail(s) is the most important factor.
If you are overclocking the i5-2500K you will need to add the appropriate amount of amperage to the combined +12 Volt continuous current rating based on the magnitude of your overclock.
The OCZ ModXStream Pro Series 600W (OCZ600MXSP), with its combined +12 Volt continuous current rating of 42 Amps and with one 6-pin and one (6+2)-pin PCI Express supplementary power connectors, is insufficient to properly power your system configuration with two Radeon HD 6870 graphics cards in 2-way CrossFireX mode and with CPU overclocking.