|Is the link below for your power supply? I asked for model number but you didn't provide one so I am not sure. |
If this is your power supply that unit has 2-+12V rails. 1@15A & 1@18A. One is dedicated to the CPU and the other would handle all the other 12V demands. Which is which I don't know. Taht said, this is not a good PSU. Only 70% efficiency and less than 20A per rail.
If you restore the computer back to all the original parts and it then runs OK, I would feel confident stating the problem is the PSU. That assumes you are fully inserting the card and you are NOT getting any video at start up.
I forgot to ask which port on the new card are you trying and are you using any adapters in the cabling?