2 burned mobos

Hi! I had burned 2 mobos in the last month and I reached the conclusion that only my XFX nvidia 8800 GTS extreme si the cause. I want to make sure, that's why I write this. First I bought the card and installed it. THat's when problems started to come. I have an intel mobo with a p4 ht 3.4 GHz on it. The system started to shut down and not power up anymore. After reseting the cmos sometimes it powered back on until one day that nothing worked anymore. At that time I had a 400 W power supply. After troubleshooting it (taking out all the components and left only cpu and mem) I though that the power supply was not enough. So I bought a 550 W thermaltake. The computer still didn't POST even with CPU and RAM on the mobo so it was clear that it was burned. After buying a new mobo (asus p5n-sli) I installed the system back on and everything was running fine but after a while the same shutdown and not boot up thing started again. THat's when I took the video card out and used an older one in order to be able to use my computer. Today I wanted to play a little and I inserted the card back in the system. The fans did spin for one second and that was it: my computer is dead again. I just left only the mobo with the CPU on to see if it POSTs but nothing is happening. Is clear that once again I have burned the mobo. I am insanely pissed and the only explanation is that the video card is causing it. Is it a fault card? I know the post is long but that's the history of this issue. Thank you!
(I want to mention that I have in my system p4 3.4 GHZ, 1 GB ram, 3 sata HDD, I DVD-RW, one ethernet, one audigy, and the 8800GTS extreme...is it too much for a 550 W PSU?)
thank you for your suggestions.