For a system using two Radeon HD 6850 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 39 Amps or greater and have at least two 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.
The Antec Neo Eco Series 520W (Neo Eco 520C), with its combined +12 Volt continuous current rating of 40 Amps and with one 6-pin and one (6+2)-pin PCI Express supplementary power connectors, is borderline sufficient to power your system configuration with two Radeon HD 6850 in 2-way CrossFireX mode.
40 Amps should be enough to handle the graphics cards when they're manually overclocked to 920 MHz for the GPU and 1140 MHz for the memory and running Furmark. If you're also overclocking your CPU then it may not be enough depending on which CPU you have and the magnitude of its overclock.