along the lines of totally killing power... in my experience, sometimes you have to turn the switch off (if it has one) and pull the power plug. wait a few minutes (up to 30 if you want to be super anal, 2-3 should do it though) and plug it back in. a step beyond this is pulling the cmos battery - not that it will hurt anything but I wouldn't pull it unless you have to.
Also, don't just buy any ole' 400w power supply... buy a quality one. I trust a 350w antec truepower more than I'd trust a generic 550w. yes, seriously.
I have this running a (carefully) OC'd machine, a couple hard drives, a couple burners, four usb ports, etc etc and the rails are rock solid. not that I'm really stressing it though:
http://www.newegg.com/app/viewproduc...103-908&DEPA=0
though I think you'd be perfectly fine with this:
http://www.newegg.com/app/ViewProduc...103-913&depa=0