It actually turned out not to be the PSU. The fan vibration caused one of the cards to list to the side slightly and the grounded mounting bracket came into contact with the solder bumps on the back of another card. I was able to reproduce the sparking, so I'm sure that was the problem.

The Rosewills have been great to me so far.

All the rest of my hardware is fine, too. I just need to mount my cards better.