with the analogue connections, there should be some difference in SQ or PQ - this would probably be evident on PQ if you're still using those RCA cables for video, but in my experience, very hard to discern on SQ ... i haven't tried the most expensive cables (yung pwede ka na bumili ng second hand na kotse), but i'm using relatively expensive interconnects and speaker wires on my setup, but really could not discern any difference on SQ
with digital there should be no difference on PQ and SQ (e.g. optical cable or HDMI) ... it's either the cable doesn't work or working
but the big difference with more expensive cables is the reliability and durability of the cables - doesn't corrode and hardly any lose connections over time of use ... plus the pogi factor, especially if you are showcasing your system
anyway this is just my opinion and base on my experience on the cables i used ... and for reference, i am using Monster interconnects and HDMI cable, and Ixus fiber optic cable, for speaker wires i'm using the wires that are roughly PhP500 per meter - wala talaga ako marinig na difference doon sa ordinary cables, but i'm happy with those cables, yung mga ordinary kasi mabilis mag-corrode damay pa yung mga kinakabitan na ports sa receiver/amp at mga audio/video sources