on a quite 300watt power supply?
Do you mean quiet, as in low noise, or approximately 300watt? (not intending to offend or anything, some people have poor english, and some people just have typos Just want to make sure I'm understanding the question)
I have an Enermax 430watt and it's very quiet and also very powerful. It's also a very good quality supply, and they really aren't that expensive.
I would try the Enermax 330watt Whisper series if I were you. It should be quieter than my already very quiet 430watt and 330 is enough power for most PCs.