I've found a website on the internet that predicts how much power your PC needs to function correctly. It specifies the maximum power needed if all your hardware functioned at the same time. Remember, this is only a prediction so take the results with a pinch of salt. It is quite interesting though. The PC i'm building is predicted to need 225 Watt, and my PSU is only 235W. Oops, I think i better get another PSU! You will find the tool down the right hand side of the page: http://www.jscustompcs.com/power_supply/
That's pretty good actually, as it specifically notes the power supplies 12V rail is key, as well as that wattage doesn't mean a thing. Of course, if your PSU's total wattage is barely covering the maximum power output, then, yeah, it's time to spend some cash and get a nice unit. (Fortron's AX450 is just under $50 at Newegg and has dual 18A 12V rails!)
I used this site a long time ago, it's a cool idea but not very good in practice, I can't get a reading on my computer because the 6600GT (AGP8x) isn't there. With my old graphics card (FX 5200) I 'needed' 282 Watts...
