Computer freezing

I'm guessing the RAM is bad. Download MemTest86+ from memtest.org, burn the ISO image to a CD, then boot from the disc. Let it run overnight to full test the memory.

This could be any number of things. If the overnight Memtest86+ run comes back with no errors then I would next focus on either an overheating problem, faulty hard drive, or a dying power supply. What exact RAM do you have? Have you manually set the RAM speed/timings/voltage to their rated specs in the BIOS? That's very important, especially when running four sticks of RAM.

What are your idle/load CPU temps?

What HD do you have? Your manufacturer should have a drive utility that checks for faulty sectors.

What exact PSU do you have? I'm not aware of any quality units that have a 680W PSU. A POS PSU will cause all sorts of stability issues.