The first signs of an underpowered psu will be system instability, but unless you're running a peltier or a large array of hard drives 300W should be plenty. I'm running a duron at 1.8V (that's my new max voltage after failing the A7V mod :-( ), three 120mm 105cfm case fans, a hard drive (only 5400rpm though), cdrw, dvd drive, and a GeForce DDR. No problems
