Most compatible speaker/interconnect and power cables with ARC ref system?

To ME, it sounds like you don't really know what you want.  Here's a recommendation.....Call Paul at Clear Day cables.   He will send you his cables for audition....free of charge.  When listening you will have a base line of reference, for any other cables you might want to try.  ...I have had success with Audioquest, Kimber, and Wireworld.....  Remember all cables bring something different to your system.  The goal is to make you smile.  Re: Power cords.   In my estimation, power cords are the most important cable in my system.  I strongly suggest you make your own.....Get top of the line Furutech, or Oyeida IEC and wall plugs, and in 10 minutes you will have power cords as good (probably better) than any commercial cord.  Touch base with VH Audio for the parts and for his recommendation..good guidance.

Is it best to choose one manufacturer for pc, interconnects and speaker cable (the whole is better than the sum of its parts type of argument) or should each component be tested one at a time and each one be judged on its merit?

This is a common question, which will always provide controversy. Ultimately, you pay your money, so you will have to answer this question for yourself.

I know some folks who have had success running all the same brand of cables/cords. That has never worked for me though, and I have tried a LOT of cables cords.
For me it seems that the more I get of one brand, the more I become aware of the flaws of that particular cable line.
I have had better success balancing the sound to my tastes by mixing and matching.

Good luck.

Thanks again for all the advice - unfortunately as I am from South Africa I have very limited access to the variety of components that you all do, and that is why it's so great to chat to you guys, 'cos you give excellent advice based on real experience. I will just do lots of reading of the forum, and if anyone has a good experience with a component, I would love to hear about it. I am going to have to go with your suggestions, and then buy online from either audiogon or the agents. Is it safe to buy second hand pc's, interconnects and speaker cables from audiogon's site, and is it to be recommended to buy these second hand?

Shunyata would be my choice would look at power cables and their line conditioners, amazing stuff.