i tried some configs on this and i would guess that 450w would be enough without video card, 600w with
Something like a Radeon HD 6950? At anyway, AFAIK an oc'ed i7 with a pair of disks and sticks won't pass the 150-180W DC marks ever (with the IGP).
what happens, for instance, if the wattage draw is higher than what the psu is rated for?
I mean it depends of the specific PSU: some can get away scot-free when overloaded, some other else just melts. Often, at anyway, there are over-current and over-temp protections which switch off the PSU itself.
To the first type belong the Seasonic X-400FL and X-460FL, I guess.