Hmm, for PSU and casing i learnt mine more from experiences,
when buying PSUs look at the side of the box and look how many
amperes it can take, on which rail.
Since Power = Voltage x Current, and Power is measured in watts,
voltage in volts and current in ampere( written in I),
we can say Power = V x I
And then look at the powersupply boxes(multiple rail for this
eg) and see which one 12v rail can take how much current, since 12v
is a constant, it is up to current, lets say the 12v rail can take
18 amps max, using P=VxI, we get P=12 x 18, giving 216watts MAX on
that rail. and see how many rails the psu have
so u can do a bit of calculation and see if the box is crapping
or is real when they say 450watts max* or 1000watts max.( This is
like the cheap audio system which writes 5000watts PMPO, what we
want is Root Means Square value, which can be calculated by
dividing the peak value by root 2) The calculation may not be
accurate because there are still other factors which are not
considered
And depending on how your system is configured , you can
consider whether you want a multiple rail or single rail PSU
Watts is one thing, and how clean is the power produced is
another, cheap power supply may produce 500watts, but the dc
voltage(which ideally should be either one straight line of power
or no power, but impposible in real life) they produce may have
more ripple(because they are stepped down and converted from AC)
than a better quality 500watts, which have more capacitor to filter
them
Casing also learnt from experience, some casing which look
perfectly normal, but uses metal of poor quality, and you can
actually 'flex' the casing. Then i dont sell this to customers who
use graphics card which are heavy,as if u accidentlykick the PC,
the casing will 'flex', and when taht happens parts connected via
PCI and secured to the casing will 'move' along which leads to not
so good contact point at the pci slots, and comp will crash.
You know pci slots cards all have an L shape which allows
you to secure by a screw to your casing. When inserting you need to
fit the end of the longer side of the "L" shape metal piece into
the casing, and some casings do not have a cutting large enough at
the end to fit the whole card in, and you need to dig it wider with
a screwdriver
But then some casing looks small, but they take into
consideration tat u use big cards, so they actually cut away the
5.25 or 3.5 inch bay to make room
some also have crappily drilled motherboard stand off holes
which you need to use a spanner and forcfully turn that stand off
in, but that is rare cases and mostly happen on cheaper casings
