My computer has Windows 8 Basic, an Intel i5-3570k OCed at 4.2Ghz, 1000watts powersupply, 2 radeon 7950 graphics cards (crossfire, OCed to 1500Mhz), 16gb DDR3 memory, and a 1TB hard drive. For a while I could play games like DmC, the HL1 and HL2 games, Bioshock Infinite, but ever since I've been playing Sniper Ghost Warrior 2 (which I uninstalled for the following reason) my computer has been freezing 10 minutes into pretty much any game. No BSOD, just a straight-up freeze.
Have you tried reducing the overclock on your GPU's? I would start by returning them to the stock clocks and then running a benchmark and even play some games. If the problem persists you can start looking at other causes but I had a similar issue when trying to overclock my single 7950 too high without enough stability testing.