Many questions:

1. Will it boot up in safe mode (F8) ??
2. When you opened it to replace the graphics card. Was there any accumulation of dust?? I have seen a machine do this due to dust.
3. How many sticks of ram do you have? If more than one , have you tried to boot off of each one separately, after being reseated??
I'm not saying, it's not the power supply. Just some more things to check.