it's the extra 4 pins for the PCI-e power that has melted, the machine still powers up ok, but it keeps turning itself off after a few minutes, thats when the burning smell starts,
I don't have onboard graphics, I have 2x ATI 4890 in crossfire configuration.
I'm pretty sure the sound card did this, because it was acting up a few days ago, in which I pulled it out (with the machine off) and cleaned the contacts, then it seemed fine.