Shiny copper looks great end of. Only thing that could cause you issues is a large plate of - sat GPU backplate - if nicely shiny could act like a mirror and sort of ruin the lighting effect. Might be fine depending on the angle, just something to think about.
Another thing to think about is making it stay shiny. Would it be worth putting a coat of clear varnish on after its polished to a suitable shine?
Something I read might make you grin. Someone was talking about copper tubing (advanced, difficult, not for beginners...so we'll done!) and they mentioned that because the copper piping is a good conductor of heat, it should, to a certain degree, act like additional radiator...which is a nice bonus