redsnail,
That is a lot of hardware for a 300W Power Supply. The GeForceFX alone will use 75W of power under load. (It looks like you have been running underpowered for a while now.)
You would want a Power Supply that has at least 28a on the 3.3v RAIL and 17a on the 12v RAIL (Since the GeForceFX uses the 12v RAIL which is plugged directly into the Video Card.) and a Max Combined Wattage of at least 200W.
Without figuring fans, network cards and any other hardware your computer has, the
Power Supply Wattage Calculator says you need a minimum of 318W. (I would look for a good Antec Power Supply with at least 400W.)
The KT4AV has a Diagnostic LED what do the LED codes say is the problem?
(You can find a picture of the D Bracket and a list of the LED codes on Page 1-6 of your Motherboard Manual.)
Take Care,
Richard