Their total system power on CF 6850s and an i7 975 OCd to 3.75 is 393W at full load using Furmark; btw Furmark has been assessed as creating unrealistically high power consumption during tests. If you use the 393W measured at full load, then your at 63% capacity on the Seasonic 620W. 63% is pretty much in the sweet zone for that PSUs efficiency numbers and you have wattage room to spare.
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 35 Amps or greater and have at least two 6-pin PCI Express supplementary power connectors.
Overclocking of the CPU and/or GPUs will, of course, increase the +12 Volt continuous current rating required to properly power the system. I would recommend at least 40 Amps or greater on the +12V rail(s) to properly handle your overclocking.
I wouldn't recommend the Thermaltake TR2 RX 750W (W0382RU). Its high ripple and noise will rapidly degrade the electrolytic capacitors on your graphics cards and motherboard not immediately but over a period of a few months.
The SeaSonic S12II-620 Bronze, with its combined +12 Volt continuous current rating of 48 Amps and one 6-pin and one (6+2)-pin PCI Express supplementary power connectors, is more than sufficient to power your system configuration.
That stock Xtreme Gear PS600 600W PSU that came with your CyberpowerPC is a piece of junk.