The 'title' wattage of a PSU is not a guide to its capabilities; in the case of the K7S5A for example, there are "400W" PSUs that cannot support an average system which your Antec might. The 'title' wattage is solely up to the 'optimism' of the marketting department.
The PSU outputs power on three lines (rails), +3.3, +5.0 and +12.0. The amount of power available on each rail is dependant upon the power being drawn on the others, and it is not a straight-forward trade-off.
A certain amount of power is available only for the +12 rail. Unfortunately, many so-called 'high-power' PSUs claim this status by pumping this rail which is used to power motors. As Slau mentioned, the major power draw is in the acceleration of the platters; once spinning, the requirements drop dramatically.
The real item to regard is the combined output of the +3.3 and +5.0 rails (the TCO), as this is what powers the electronics. The question of which rail is used is somewhat up to the motherboard manufacturer, as is the efficiency of the board's voltage regulation.
A 'good' PSU will specify its TCO and the amperages available on the two rails as a min-max/max-min relationship.
A TCO of 160 should power a minimal system, 200 the average and 240 a large one.
To give a real-life example, I replaced an Enermax 465whatever (so many variants depending on fan control) with a Topower 350; both fail when I try to O/C my system to a FSB of 147.
In spite of the difference in 'titled' power, both have a TCO of 220W.