Takes like 5 mins to take the gfx card out and test so you may aswell, but I highly doubt its the source of your problem.
I had a similar issue when I built my pc, where it would power off straightaway. The problem for me was power related unsurprisingly, with a small bit of the metal from the I/O shield touching the inside of a port and short-circuiting the mobo (not easy to spot). The fact your pc had been working means this is probably not your problem though.
An easy thing to check would be that there are no loose cables/connections inside your case aswell.
If its not posting it could also be mobo/cpu which are pretty hard to test unless you've got a spare mobo laying around.
Doesn't Setsul live in Germany? just summon him with the ritual