cable does make a difference..
its very simple:
low current signals, like the ones from your pc to your amp, dont need to be in shit hi quality, couse it doesnt drain much current, meaning the resistance in the cable dont have much to say. although, it is good to have a grounded signal cable if you got interference.
a high current cable, like the one from your amp to your speakers, can give better results if it is of a larger diameter, hence, is off a lower resistance than a smaller one.
U=R*I
where U is the Voltage, R is the resistance and I is the current.
since U=R*I;
R=U/I
and
I=U/R
R is then the resistance in your cable, and also in your speaker, if your speaker is 8Ohm(Resistance), and your cable is long and thin, making it also 8Ohm, you will get a 50% effect to the speaker, couse both the speaker and the cable is draining as much current.
(note: this is not 100% true, since a soundsignal is a alternating current, and not a direct current (battery, constant 12V for example.))
U is the Voltage from your amp, a US outlet is 115V, a Euro outlet is 230V, depending on the power of your system, a signal can vary from some Volts to hundreds..
I is the Current, simple said, its the number of electrons flowing thru the system in 1 second.
so as a thumbrule:
for a signal cable, diameter is not so important, grounding and shielding is
for a speaker cable, diameter is somehow important, specially if you got long cables or a powerful speaker, grounding and shileding is not so important, since its of a higher Voltage, noise and interference is not hearable if you play it loud.
but if you are one of them uberhifi geeks, go buy the largest cable you can find