...and it came out to 423W with 20% capacitor aging. So, what happens if I plug in a few extra hard-drives and my computer tries to draw just a little over what my PSU can put out? Does the computer just shut down or can I fry something I'd rather not?
Will I be safe or am I resigned to buying a bigger PSU sometime soon?
The calculators over-inflate EVERYTHING so you go off and buy a bigger PSU than you need.
Esrever is right, you will be fine - Dont ever cut it too fine though, gather some knowledge before buying a Graphics card, but use benchmarks and recommendations from us lot rather than Calculators and Vendor requirements.
I personally will never go below 850W, I like to be prepared for any event (Like a promotion which = another PC component xD )
20% capacitor aging grossly inflates things, your PSU might lose 20% of its capacity after about 10 years, its a good unit and isn't going to age that poorly for quite a while, you are fine with that PSU in that system for quite a while.