I usually look at the PSU when problems occur after GPU upgrade, but yours looks ok, Seasonic 620W should hold up just fine.
I also should probably mention I had to move the ram to the far 2 slots because the cooler was covering the 2 close ones.
Now that's some really good info, if you moved your RAM into different slots that could cause instability. Generally it's much better to have the RAM in dual-channel slots, that is using same color slots. In your case, if you put the RAM sticks one in a black slot and the other in a blue slot that's a no-no
Try changing your RAM into the black slots and use Memtest on your system bare bones to help you isolate the problem.
Check out these links:http://www.hardwaresecrets.com/article/ASUS-M4A89GTD-PRO-USB3-Motherboard-Review/938/4http://www.memtest.org/