There's a lot of people beleive a 300W supply can take it. With many added cards and devices I figure 350 is nearer the mark, but I used a 400W for one I made, just to make sure.
Again, try it. If it works you haven't wasted any money.
I purchased a PC case with 300W power supply. I'm running 4 PC case fans, Volcano 7 heatsink fan and a Soyo Dragon Plus card reader. It's working fine, knock on woods.
From what I've read and seen personally, the quality of the PS matters as much or more than the wattage. I'm running an XP 1700+, 2 7200 RPM hard drives, a CD burner, DVD ROM, GF2MX400, sound card, 2 NICs on a quality Antec 300W with no problems.
If your 300 watter is good quality, you probably shouldn't have any problems.
Trying it is the only way to know.
Good Luck.
"There's no such thing as gravity, the Earth just sucks"
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