TIMTS,
It would help to have the full specs on your comp including all AGP and PCI Cards.
As for the 400W Power Supply, please give the amps on the 3.3v, 5v, and 12v RAILs, also include the Max Combined Wattage of the 3.3v + 5v RAILs. (What brand is the PS?)
Then when you mounted the Heatsink, make sure it is mounted correctly:
and tell us what kind of Thermal Grease you used when you mounted the Heatsink.
Take Care,
Richard