Advice re. broken 8800 Ultra

Which is more likely to cause the green and pink dots problems on a screen resulting in a broken card. Card in question is an 8800 Ultra. I had to take mine back to change it yesterday.

1 - Installing the wrong drivers (Windows XP) into a Windows 7 installation. Caused BSOD, instant crashes. Stupid mistake but I'd spent all day on the machine trying to get Windows 7 to work properly, taking drivers off, re installing them etc etc. Just went into autopilot mode and made a silly mistake.

2 - Using a cheap power supply (800W). I have one PCI Express connector for the 8800, but the other one I am having to use is a PCI Express connector from a 2 '4' pin molex connectors.

Option '2' didn't cause any problems, but the instructions with the card say it is a bad thing to do. If it is so bad, why are the connectors on the market?!