I once bought a 8800 GTX. I wanted it to run cooler, so I bought the 2-part cooler from Zalman that had a separate aluminum heatsink array for the memory and whatnot, it looks like this when installed:
The aluminum part comes with brand new thermal pads, so I thought I was good to go. After installation, the card operated for about 10 minutes and then keeled over. Upon disassembly, I discovered that on just one of the memory chips there was a tiny bit of glue residue that caused the new thermal pads to have poor contact and so it became the weakest link.
Let my dead 8800 GTX serve as a warning to clean off your memory properly! Then clean it again once it's clean!