Funny Graphics Card Issue

A few months ago I upgraded from a X1800XT to a HD4870 512MB. My PSU was unable to support this, so I upgraded to a Be Quiet! DarkPower 650W. I am running a Q6600@stock, 4GB of DDR2-800 RAM all on a ASUS P5B Pro .This is where it gets weird: since then, I've had to graphics cards die on me. The first one died the moment I turned on my newly-upgraded PC for the first time. That was understandable, seeing as it had just endured a particularly rough plane trip; perhaps it had become damaged on the way. I ordered a replacement, a Sapphire HD4870 1GB, which worked fine for a few weeks... before dying again in a most peculiar manner. It died without warning while I was checking my email. A few hours later, it booted fine, no issues. Shortly after that, it died again - permanently. Could this be due to a defective PSU?