It might not be the power rating, but the NOISE LEVEL. You don't seem to understand that the voltage level is not constant. Let's take the 3.3v line for example, since that's the one that powers the CPU, RAM, and video card:
Power supply A) 3.10 to 3.60v swinging at around 60Hz.
Power supply B) 3.25 to 3.35v swinging at around 60Hz
Now, it wouldn't matter much if Power Supply A was 500W and Power Supply B was 300W, B would always result in a more stable system than A.
It takes very large capacitors to stabilize a power supply. Good power supplies might have $10 in capacitors, while cheap ones might use $2 in capacitors. So obviously the manufacturer can use higher quality parts with a $10 budget than with a $2 budget.
I don't know what size your power supply is right now, but if it follows a scenario similar to A above, it's going to give you problems. I've seen even low end Antec power supplies with this problem.
The new high end Antec True Power power supplies are supposed to be more stable. As are Enermax. I hear Sparkle is a good brand. And there are a couple top brand Japanese companies I can't even remember the names of.
I recommend 350W in a HIGH QUALITY power supply.
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