|I took another look.|
Apparently the C521 optionally came with a X1300 or X1300Pro
Dell Tech Specs
On that page it lists only one power supply capacity - 280 watt. I searched on the web again and found no power supply for C521 with more capacity, so there seems to be no undocumented power supply for the C521, which sometimes iis the situation, that has more capacity.
I looked again on the web using: X1300 minimum watt
and I found many for X1300Pro that say it requires 350 watts, but for X1300 or X1300XT they quote either 250 or 300 watts.
So - if your new cpu has very similar current requirements to the one it originally had, the power supply should have worked with your X1300, assuming it wasn't a Pro, but the power supply would have been running at 100% of it's capacity, or nearly so, or 89% or it's capacity, or nearly so, all the time.
Since the max capacity rating is almost always an intermittent rating, and it's recommended you do NOT load your power supply to more than 80% of it's capacity continuously, it's just plain unethical for Dell to use a 280 watt power supply when the C521 has a X1300 or X1300 Pro installed.
I think what's really doing on here is the power supply is no longer capable of putting out 280 watts, for whatever reason - I would think it's because it was being loaded to more than 80% of it's capacity continuously .
I suspect that if you got another video card that requires a minimum 250 watt PS capacity, you're going to have the same problem you had before.
What was your original CPU ?
"i just upgraded with an AMD Athlon 64 X2 5600+ "
According to this it draws 65 watts: