Vdosc,
I love your desktop system :)
Great question and one that is endlessly debated.
Frankly, it depends on your system. My experience has been that, in general, changing USB cables, and to a smaller degree changing Firewire cables, in higher resolutions systems makes more of a difference than in lower resolutions systems. Additionally, changing a USB/FW cable can be make more of difference if it is the weakest component in your system. The difference is very dependent you your system but cables do make a difference.
I have a comparison that I like to use to demonstrate audible USB cable differences. The comparison involves a Belkin 5 meter printer cable and a .5 meter Kimber Kable - AG - Silver - USB B BUS cable. Switching between the two cables in my system produces obvious audible differences. That may seem unfair and a no brainer but the point is to demonstrate that different cables can produce different sound. After accepting there can be differences between cables, it's then just a matter of the degree of difference.
There is definitely a point of diminishing returns so focusing on the quality of materials, connectors and workmanship should be top considerations. After that, it's just tone control.
Good Luck,
Greg