The single most cost effective improvement I've ever made was the addition of a good power conditioner. But Elizabeth is right about there being so many variables and we don't even know what your budget is. I concur with the Venom 3 recommendation because I think it's the best $90 power cable you can buy.
One bit of advice if you experiment with PC's: Try to get loaners that are burnt in for you. (100 hours or more) Then, let the cables run for a day on a component before you switch....quick switching is misleading and can lead to severe audio nervosa. ;-)