In theory 500W is not really enough for that system, no matter how good quality the PSU.
Advertised TDP is 170W per GPU for GTX560TI and 130W for the QX6850. That's 470W without overclocking, leaving you 30W for everything else. 30W barely covers the losses in the motheroard VRM, nevermind power consumption from the chipset, PCIe bus, RAM, HDD, fans, & networking.
In practice you'd probably be "mostly ok" at stock clocks since you are unlikely to fully load CPU and GPU's but overclocking is verboten.