no doubt, copper is indeed capable of storing a lot of heat in it.
as compared to Alu, copper sinks will help in lowering the load temperature a lot !!
i'd say to go for it, but without doing any damage to the hardware and improve the airflow around that area:lol:
come on man, dont make me laugh......i dont wish to comment about this, but some people are so noob and they just simply make the wrong decision all the time.

have you considered that he's using SLi setup man ???
he's just wasting his money on the 2x fan cards because there are simply no space for air circulation in between the two cards.

fan card is definitely a NO !!! for SLi setup !!!!

================== 7900GTX(1)

fan card 1

================== 7900GTX(2)

fan card 2
* fan card 1 will suck hot air from the back of 7900GTX(2) and blow it on 7900GTX(1)
* hot air from fan card 2 will recirculate back to fan card 2
* the fan card will block air from coming in from the side

graphics card releases heat from all direction.
the best way to cool down a SLi setup is still blowing from the side, or blowing from the end of the card to and exit through the PCI holes.
i seriously felt sorry for him for wasting his cash because he had listen to your solution  

Aluminium is better for passive cooling. If u opt for copper ramsinks u need to use a low cfm fan to benefit it. If not u might suffer degradation since the temp on the ram's surface would be higher. This is tested before, aluminium is good for passive and copper would be for active cooling. If you just changed it to copper without fans its going to be a bad idea. Trust me.

i am not sure about this...but i did try this method on some of my old VGA's...they were Nvidia TNT 2 and Voodoo 3......

i used the hair dryer...took the card out and set the blower to hot...it worked for both the cards...well just a suggestion...

works with motherboards too i guess~
Edit: can use something to blow air in? from the side casing?? (Just suggesting..could work? for SLI setup?)