Where do cables upgrades have the most impact?

Get some really good interconnects and then a decent power cord, or two. Speaker cables will fall into place much easier and for less cost.

All the best,
Nonoise

I switched RCA analog interconnects on my FM tuner and couldn't believe the difference from inexpensive Acoustic Research cables ($15 or so) to some entry level Audioquests ($35 or so). Not night and day, but noticeable. Same when I de-soldered the horrid RCA's on a Project turntable and installed gold plated female RCA jacks and ran some other entry level Audioquests. Not night and day, but noticeable and no hum! I frankly can't hear any difference with digital cable upgrades. 

Well...after reading everyone’s responses what I surmise is that opinions differ because everyone’s system is different and cable upgrades impact systems differently. So...the old audio adage of you have to try it and see how it sounds to you in your system in your environment holds true with cables as with almost everything else in audio.

yep

Most online cable dealers offer a trial period included with a sale.
So, the next step is to decide on which brand to try.

I would advise introducing one cable at a time when auditioning.