I found the biggest improvement was on the power amp. If you want to upgrade one PC at a time - which is the wise way to do it, start on the power amp.
As you work through the components the improvement will become less and less, so I would suggest get a good cable with good current capacity. (Don't skimp) Also check with the manufacturer what lengths they recommend. I found with Wireworld cables the longer the better, but the opposite some others.