I have one of those PC Power & Cooling Silencer 750 Quad's. Purchased it back in 2007 and it died three years later. I wasn't even drawing any more than 275 Watts from it. I had it RMA'ed under warranty.
For a system using a single Radeon HD 7850 graphics card AMD specifies a minimum of a 500 Watt or greater power supply. The power supply should also have a combined +12 Volt continuous current rating of 24 Amps or greater and have at least one 6-pin PCI Express supplementary power connector.
For a system using two Radeon HD 7850 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 32 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 PC Power & Cooling Silencer 750 Quad (S75Q), with its combined +12 Volt continuous current rating of 60 Amps and with two 6-pin and two (6+2)-pin PCI Express supplementary power connectors, is more than sufficient to power your system configuration with a single Radeon HD 7850 or even two Radeon HD 7850 graphics cards in 2-way CrossFireX mode.