Is it just wasteful/unefficient, or is there another reason not to get a killowatt PSU? Also, is the "Estimated Wattage" of 270W on that link at the top of this post safe to go by, or how much 'wiggle room' should I look for? Finally, do the volts matter or just the wattage?
Wasteful/inefficient indeed. If not gaming or creating/editing stuff, most of the time your computer will spend idling around. Power supplies are designed so that their efficiency is at its peak at medium loads and can vary by up to 5% at idle/max points. If you leave your PC running 24/7 and it has inefficient PSU, imagine forgetting to turn off the desk lamp besides it.
PC Part Picker's maximum value is amazingly on spot - it uses TDP ratings of components. Mind you, TDP isn't peak consumption, but continous maximum for safe operation (without aftermarket parts). To maintain wiggle room and OC, this value should be 60%. Which does give us 430W, not coincidentially.
Volts never matter - volts don't change. Watts do, basically. With CX430 (very good unit for its price, but with gimped +12V) amps come into play. Limit of 28A prohibits any notable upgrades (for example, swapping that GT640 for something with more punch down the road).
Do note you do need an aftermarket cooling solution for any meaningful OC. Haswell tends to run a bit hot even idling around. Pity it'll dent your budget even more, though - 4670K would've prevented that, but what's done is done.
It probably would be, but chances are I won't be purchasing another processor for a long time, so it's really more a matter of future-proofing it (maybe I get real interested in overclocking or gaming or something and I regret not spending the $50 more).
Just to confirm, 1333 would (technically, if I were to overclock it) be slower than 1600?
There's another parameter (four, in fact) which is often not mentioned - CAS latency (CL), which, contrary to clock speed, lower is better.
But your selected Patriots are dead cheap, so no use shaving nanoseconds for leet points. You'll enable XMP profile and forget about it. RAM is the fastest storage already.