wow...dude...this scenerio sounds creepily familiar...
I, myself, had a 300W generic with an oc'ed 1600+ and GF4, bust at 150 ^_^...2 40gb ibm's drives in raid 0, 30gb boot drive, 2 cd-roms...and to cut to the chase...mine did a similar thing...where i had the fans partially spun up...and nothing else. I determined, by yoinking a psu i had laying around, that that my 300W was a dud... after a few good months...it failed me...
I wont SAY FOR SURE that it is the psu in your case...but i would recommend that you borrow another psu, and try it out, if it works, great. if it doesnt, come back to the forums, we wont fail you! lol Experimentation is the only way to go.
and yeah, - 5% on any line is a good sign of ineffeciency, and for overclocking, 300W is almost never enough, i have an Antec 430, and some people still prefer higher.