i'm an experienced IT member, and i've noticed there's always somekind of "hiss/background noise" when a pc/whatever sound is connected to a high end av receiver/amplifier... SO my question is : is it better to get an external audio card to bypass the electricalmagnetic field caused by all the components inside a PC ?
No, it won't really help. The construction of the PC is such that it is really not suited to professional audio reproduction, although the hi fi audio storage capability IS there.
There are certain motherboards which are intentionally made for a better signal to noise ratio. These are marketed as "home theater PC-multimedia" boards.
They have a much lower noise level, and a much higher price, right about double or higher.
But, that's what you might want, if you are seriously into it.
Also a better power supply and a cable isolation transformer (for cable internet) will reduce hum and some other common problems.
Hum from a ground loop is the most common noise that will result when connecting PC to Hi Fi...which is cured with the cable TV isolation transformer.
If you want better quality and don't need surround sound try a pro-audio board like the ESI Juli@. It has balanced and unbalanced analog outputs as well as optical and coaxial SPDIF digital outputs. The optical output will provide some isolation between the PC and the receiver.