Sisoft Sandra benchmark on video card and card goes kaboom.

Long time reader, first time poster here....

Okay, here's the issue. New build, except power supply. Power supply was previously known to be working fine in a similar system. 500w, no stability issues (say with occt or prime 95 etc).

System specs are or were as follows:

Intel i7 920 cpu (don't overclock anything currently anyway)
ASUS P6T motherboard
500w psu (unsure of brand but it wasn't one of the best - had it for years and it's now gone for two reasons, see below)
3x2GB DDR 3 ram (kingston performance series)
2 x 1tb hdd, 1 x 320gb hdd, 1 x bd-r (all sata).
And it was a radeon hd 4890 (brand: xfx)

What happened is this:

I ran sisoft sandra's gpu calculation benchmark (I think that's what it was called). About maybe 30-50% through the test, I heard a quite loud bang and several loud cracks. Power shut off immediately and my UPS changed status too. UPS appears fine (though a bit worried too about that). I unplugged as quickly as I could and turned off power supply too. Removed the power supply.

I had a spare PSU that had enough power to test bare components (mostly to see if cpu and mobo would try to post - thankfully it did; beep etc). And nothing else was damaged, either, and I'm actually writing this on the same box (different video card now though, and new psu).

So, question is this:

Sisoft Sandra itself says that if there are weak or faulty components, they can fail. Which makes sense. But, anyone ever see anything like this with any component, or even the specific: xfx video card. Recommended psu was 500 or greater watt psu. And as far as I know, it was in okay condition. Also, pretty sure that not enough power will equate to stability issues, rather than what I experienced.

Of course, if the psu happened to die it could have taken out anything with it, but the fact i was doing a gpu benchmarks makes me wonder what really was the cause. Granted, the psu did die in the process and I now have an 850 w (thermaltake) but I am set back to my old geforce 8800gts 640mb (which isn't bad, but I was enjoying the better performance out of the 4890) ...

One more thing. I see no burn marks on the motherboard. And also no melting or anything. Right now my current video card is in the second PCI-E x16 slot. Thing is, I wonder if the first slot is okay. Bit worried about that.

Any ideas or help would be appreciated. If I missed something, or left something out, my apologies - let me know and I'll correct. Then again, it's 4:27 and been up since 3am, but that's another matter entirely.

Thanks.